SECOND EDITION

THE

lNG

GUAGE

BRIAN W KERNIGHAN

DENNIS M. RITCHIE

PRENTICE HALL SOFTWARE SERIES

THE

PROGRAMMING

LANGUAGE

Second Edition

THE

PROGRAMMING

LANGUAGE

Second Edition

Brian W. Kernighan • Dennis M. Ritchie

AT&T Bell Laboratories

Murray Hill, New Jersey

PRENTICE HALL, Englewood Cliffs, New Jersey 07632

Ubrary of Congress Cataloging-in-Publication Data

Kernighan, Brian W.

The C programming

language.

Includes index.

1. C (Computer program language)

Dennis M. II. Title.

QA76.73.C15K47 1988 005.13'3

ISBN 0-13-110370-9

ISBN 0-13-110362-8 (pbk.)

I. Ritchie,

88-5934

system, or transmitted, in any form or by any means, electronic, mechanical, photocopy-ing,

recording, or otherwise, without the prior written permission of the publisher. Printed in the

United States of America. Published simultaneously in Canada.

UNIX is a registered trademark of AT&T.

This book was typeset (pic l tbll eqn l troff -ms) in Times Roman and Courier by the authors,

using an Autologic APS-5 phototypesetter and a DEC VAX 8550 running the 9th Edition of

the UNIXS operating system.

Prentice Hall Software Series

Brian Kernighan, Advisor

Printed in the United States of America

10 9 8 7

ISBN 0-13-110362-8 iPBK} ISBN

O-l3-110370-9

Prentice-Hall International (UK) Limited, London

Prentice-Hall of Australia Pty. Limited, Sydney

Prentice-Hall Canada Inc., Toronto

Prentice-Hall Hispanoamericana, S.A., Mexico

Prentice-Hall of India Private Limited, New Delhi

Prentice-Hall of Japan, Inc., Tokyo

Simon & Schuster Asia Pte. Ltd., Singapore

Editora Prentice-Hall do Brasil, Ltda., Rio de Janeiro

Contents

Preface

Preface to the First Edition

Introduction

Chapter 1.

A Tutorial Introduction

l.l

Getting Started

1.2

Variables and Arithmetic Expressions

1.3

The For Statement

1.4

Symbolic Constants

1.5

Character Input and Output

1.6

Arrays

1.7

Functions

1.8

Arguments-Call by Value

1.9

Character Arrays

l.lO

External Variables and Scope

Chapter 2.

Types, Operators, and Expressions

2.1

Variable Names

2.2 Data Types and Sizes

2.3

Constants

2.4

Declarations

2.5

Arithmetic Operators

2.6

Relational and Logical Operators

2.7

Type Conversions

2.8

Increment and Decrement Operators

2.9

Bitwise Operators

2.10

Assignment Operators and Expressions

2.11

Conditional Expressions

2.12

Precedence and Order of Evaluation

Chapter 3.

Control Flow

3.1

Statements and Blocks

3.2

If-Else

THE C PROGRAMMING LANGUAGE

CONTENTS

3.3

Else-If

3.4

Switch

3.5

Loops-While and For

3.6

Loops-Do-while

3.7

Break and Continue

3.8

Goto and Labels

Chapter 4.

Functions and Program Structure

4.1

Basics of Functions

4.2

Functions Returning Non-integers

4.3

External Variables

4.4

Scope Rules

4.5

Header Files

4.6

Static Variables

4.7

4.8

Block Structure

4.9

Initialization

4.10

Recursion

4.11

The C Preprocessor

Chapter 5.

Pointers and Arrays

5.1

Pointers and Addresses

5.2

Pointers and Function Arguments

5.3

Pointers and Arrays

5.4

Address Arithmetic

100

5.5

Character Pointers and Functions

104

5.6

Pointer Arrays; Pointers to Pointers

107

5.7

Multi-dimensional Arrays

110

5.8

Initialization of Pointer Arrays

113

5.9

Pointers vs. Multi-dimensional Arrays

113

5.10

Command-line Arguments

114

5.11

Pointers to Functions

118

5.12

Complicated Declarations

122

Chapter 6.

Structures

127

6.1

Basics of Structures

127

6.2

Structures and Functions

129

6.3

Arrays of Structures

132

6.4

Pointers to Structures

136

6.5

Self-referential Structures

139

6.6

Table Lookup

143

6.7

Typedef

146

6.8

Unions

147

6.9

Bit-fields

149

Chapter 7.

Input and Output

151

7.1

Standard Input and Output

151

7.2

Formatted Output-Printf

153

THE C PROGRAMMING LANGUAGE

CONTENTSrii

7.3

Variable-length Argument Lists

155

7.4

Formatted lnput-Scanf

157

7.5

File Access

160

7.6

Error Handling-Stderr and Exit

163

7.7

Line Input and Output

164

7.8

Miscellaneous Functions

166

Chapter 8.

Tbe UNIX System Interface

169

8.1

File Descriptors

169

8.2

Low Level I/O-Read and Write

170

8.3

Open, Creat, Close, Unlink

172

8.4

Random Access-Lseek

174

8.5 Example-An Implementation of Fopen and Getc

175

8.6

Example-Listing Directories

179

8.7

Example-A Storage Allocator

185

Appendix A. Reference Manual

191

Introduction

191

Lexical Conventions

191

Syntax Notation

194

Meaning of Identifiers

195,

Objects and Lvalues

197

Conversions

197

A 7

Expressions

200

Declarations

210

Statements

222

A l 0

External Declarations

225

A 11

Scope and Linkage

227

A 12

Preprocessing

228

Al3

Grammar

234

Appendix B. Standard Library

241

Input and Output:

<stdio.h>

241

Character Class Tests: <ctype.h>

248

String Functions:

<string.h>

249

Mathematical Functions:

<math.h>

250

Utility Functions:

<stdlib.h>

251

Diagnostics: <assert.h>

253

Variable Argument Lists:

<stdarg.h>

254

Non-local Jumps:

<setjmp.h>

254

Signals: <signal.h>

255

BlO

Date and Time Functions:

<time.h>

255

B1l

Implementation-defined Limits: <limits.h> and <float.h>

257

Appendix C.

Summary of Changes

259

Index 263

Preface

The computing worid has undergone a revolution since the publication of

The C Programming Language in 1978. Big computers are much bigger, and

personal computers have capabilities that rival the mainframes of a decade ago.

During this time, C has changed too, although only modestly, and it has spread far

beyond its origins as the language of the UNIX operating system.

The growing popularity of C, the changes in the language over the years, and

the creation of compilers by groups not involved in its design, combined to

demonstrate a need for a more precise and more contemporary definition of the

language than the first edition of this book provided. In 1983, the American

National Standards Institute (ANSI) established a committee whose goal was to

produce "an unambiguous and machine-independent definition of the language

C," while still retaining its spirit. The result is the ANSI standard for C.

The standard formalizes constructions that were hinted at but not described

in the first edition, particularly structure assignment and enumerations. It pro-

vides a new form of function declaration that permits cross-checking of defini-

tion with use. It specifies a standard library, with an extensive set of functions

for performing input and output, memory management, string manipulation, and

similar tasks. It makes precise the behavior of features that were not spelled out

in the original definition, and at the same time states explicitly which aspects of

the language remain machine-dependent.

This second edition of The C Programming Language describes C as defined

by the ANSI standard. Although we have noted the places where the language

has evolved, we have chosen to write exclusively in the new form. For the most

part, this makes no significant difference; the most visible change is the new

form of function declaration and definition. Modern compilers already support

most features of the standard.

We have tried to retain the brevity of the first edition. C is not a big

language, and it is not well served by a big book. We have improved the exposi-

tion of critical features, such as pointers, that are central to C programming. We

have refined the original examples, and have added new examples in several

chapters. For instance, the treatment of complicated declarations is augmented

by programs that convert declarations into words and vice versa. As before, all

X PREFACE

examples have been tested directly from the text, which is in machine-readable

form.

Appendix A, the reference manual, is not the standard, but our attempt to

convey the essentials of the standard in a smaller space. It is meant for easy

comprehension by programmers, but not as a definition for compiler writers-that

role properly belongs to the standard itself. Appendix B is a summary of the

facilities of the standard library. It too is meant for reference by program-mers, not

implementers. Appendix C is a concise summary of the changes from the original

version.

As we said in the preface to the first edition, C "wears well as one's experi-ence

with it grows." With a decade more experience, we still feel that way. We hope that

this book will help you to learn C and to use it well.

We are deeply indebted to friends who helped us to produce this second edi-

tion. Jon Bentley, Doug Gwyn, Doug Mcilroy, Peter Nelson, and Rob Pike gave us

perceptive comments on almost every page of draft manuscripts. We are grateful

for careful reading by AI Abo, Dennis Allison, Joe Campbell, G. R. Emlin, Karen

Fortgang, Allen Holub, Andrew Hume, Dave Kristol, John Linderman, Dave

Prosser, Gene Spafford, and Chris Van Wyk. We also received helpfpl suggestions

from Bill Cheswick, Mark Kernighan, Andy Koenig, Robin Lake, Tom London,

Jim Reeds, Clovis Tondo, and Peter Wein-berger. Dave Prosser answered many

detailed questions about the ANSI stand-ard. We used Bjarne Stroustrup's C++

translator extensively for local testing of our programs, and Pave Kristof provided

us with an ANSI C compiler for final testing; Rich Drechsler helped greatly with

typesetting.

Our sincere thankS to all.

Brian W. Kernighan

Dennis M. Ritchie

Preface to the First Edition

C is a general-purpose programming language which features economy of

expression, modern control flow and data structures, and a rich set of operators. C is

not a "very high level" language, nor a "big" one, and is not specialized to any

particular area of application. But its absence of restrictions and its gen-erality make

it more convenient and effective for many tasks than supposedly more powerful

languages.

C was originally designed for and implemented on the UNIX operating sys-tem

on the DEC PDP-11, by Dennis Ritchie. The operating system, the C com· piler, and

essentially all UNIX applications programs (including all of the software used to

prepare this book) are written in C. Production compilers also exist for several other

machines, including the IBM System/370, the Honeywell 6000, and the Interdata

8/32. C is not tied to any particular hardware or sys-tem, however, and it is easy to

write programs that will run without change on any machine that supports C.

This book is meant to help the reader learn how to program in C. It con-tains a

tutorial introduction to get new users started as soon as possible, separate chapters

on each major feature, and a reference manual. Most of the treatment is based on

reading, writing and revising examples, rather than on mere statements of rules. For

the most part, the examples are complete, real programs, rather than isolated

fragments. All examples have been tested directly from the text, which is in

machine-readable form. Besides showing how to make effective use of the

language, we have also tried where possible to illus-trate useful algorithms and

principles of good style and sound design.

The book is not an introductory programming manual; it assumes some fam-

iliarity with basic programming concepts like variables, assignment statements,

loops, and functions. Nonetheless, a novice programmer should be able to read

along and pick up the language, although access to a more knowledgeable col-

league will help.

In our experience, C has proven to be a pleasant, expressive, and versatile

language for a wide variety of programs. It is easy to learn, and it wears well as

one's experience with it grows. We hope that this book will help you to use it well.

xii PREFACE TO THE 1ST EDITION

The thoughtful criticisms and suggestions of many friends and colleagues

have added greatly to this book and to our pleasure in writing it. In particular,

Mike Bianchi, Jim Blue, Stu Feldman, Doug Mcilroy, Bill Roome, Bob Rosin,

and Larry Rosier all read multiple versions with care. We are also indebted to AI

Abo, Steve Bourne, Dan Dvorak, Chuck Haley, Debbie Haley, Marion Harris,

Rick Holt, Steve Johnson, John Mashey, Bob Mitze, Ralph Muha, Peter Nelson,

Elliot Pinson, Bill Plauger, Jerry Spivack, Ken Thompson, and Peter Weinberger

for helpful comments at various stages, and to Mike Lesk and Joe Ossanna for

invaluable assistance with typesetting.

Brian W. Kernighan

Dennis M. Ritchie

Introduction

C is a general-purpose programming language. It has been closely associ-ated

with the UNIX system where it was developed, since both the system and most

of the programs that run on it are written in C. The language, however, is not tied

to any one operating system or machine; and although it has been called a

"system programming language" because it is useful for writing com-pilers and

operating systems, it has been used equally well to write major pro-grams in

many different domains.

Many of the important ideas of C stem from the language BCPL, developed

by Martin Richards. The influence of BCPL on C proceeded indirectly through

the language B, which was written by Ken Thompson in 1970 for the first UNIX

system on the DEC PDP-7.

BCPL and B are "typeless" languages. By contrast, C provides a variety of data

types. The fundamental types are characters, and integers and floating-point

numbers of several sizes. In addition, there is a hierarchy of derived data types

created with pointers, arrays, structures, and unions. Expressions are formed from

operators and operands; any expression, including an assignment or a function call,

can be a statement. Pointers provide for machine-independent address arithmetic.

C provides the fundamental control-flow constructions required for well-

structured programs: statement grouping, decision making (if-else), selecting one

of a set of possible cases (switch}, looping with the termination test at the top

(while, for) or at the bottom (do), and early loop exit (break).

Functions may return values of basic types, structures, unions, or pointers.

Any function may be called recursively. Local variables are typically

"automatic," or created anew with each invocation. Function definitions may not

be nested but variables may be declared in a block-structured fashion. The

functions of a C program may exist in separate source files that are compiled

separately. Variables may be internal to a function, external but known only

within a single source file, or visible to the entire program.

A preprocessing step performs macro substitution on program text, inclusion

of other source files, and conditional compilation.

C is a relatively "low level" language. This characterization is not

2 INTRODUCTION

pejorative; it simply means that C deals with the same sort of objects that most

computers do, namely characters, numbers, and addresses. These may be com-bined

and moved about with the arithmetic and logical operators implemented by real

machines.

C provides no operations to deal directly with composite objects such as

character strings, sets, lists, or arrays. There are no operations that manipulate an

entire array or string, although structures may be copied as a unit. The language

does not define any storage allocation facility other than static defini-tion and the

stack discipline provided by the local variables of functions; there is no heap or

garbage collection. Finally, C itself provides no input/output facili-ties; there are no

READ or WRITE statements, and no built-in file access methods. All of these

higher-level mechanisms must be provided by explicitly-called functions. Most C

implementations have included a reasonably standard collection of such functions.

Similarly, C offers only straightforward, single-thread control flow: tests, loops,

grouping, and subprograms, but not multiprogramming, parallel opera-tions,

synchronization, or coroutines.

Although the absence of some of these features may seem like a grave defi-

ciency ("You mean I have to call a function to compare two character strings?"),

keeping the language down to modest size has real benefits. Since C is relatively

small, it can be described in a small space, and learned quickly. A programmer can

reasonably expect to know and understand and indeed regu-larly use the entire

language.

For many years, the definition of C was the reference manual in the first edition

of The C Programming Language. In 1983, the American National Standards

Institute (ANSI) established a committee to provide a modern, comprehensive

definition of C. The resulting definition, the ANSI standard, or "ANSI C," was

completed late in 1988. Most of the features of the standard are already supported

by modern compilers.

The standard is based on the original reference manual. The language is

relatively little changed; one of the goals of the standard was to make sure that most

existing programs would remain valid, or, failing that, that compilers could produce

warnings of new behavior.

For most programmers, the most important change is a new syntax for declaring

and defining functions. A function declaration can now include a description of the

arguments of the function; the definition syntax changes to match. This extra

information makes it much easier for compilers to detect errors caused by

mismatched arguments; in our experience, it is a very useful addition to the

language.

There are other small-scale language changes. Structure assignment and

enumerations, which had been widely available, are now officially part of the

language. Floating-point computations may now be done in single precision. The

properties of arithmetic, especially for unsigned types, are clarified. The

preprocessor is more elaborate. Most of these changes will have only minor

THE C PROGRAMMING LANGUAGE 3

effects on most programmers.

A second significant contribution of the standard is the definition of a library

to accompany C. It specifies functions for accessing the operating system (for

instance, to read and write files), formatted input and output, memory alloca-

tion, string manipulation, and the like. A collection of standard headers pro-

vides uniform access to declarations of functions and data types. Programs that

use this library to interact with a host system are assured of compatible behavior.

Most of the library is closely modeled on the "standard 110 library" of the UNIX

system. This library was described in the first edition, and has been widely used

on other systems as well. Again, most programmers will not see much change.

Because the data types and control structures provided by C are supported

directly by most computers, the run-time library required to implement self-

contained programs is tiny. The standard library functions are only called

explicitly, so they can be avoided if they are not needed. Most can be written in

C, and except for the operating system details they conceal, are themselves port-

able.

Although C matches the capabilities of many computers, it is independent of

any particular machine architecture. With a little care· it is easy to write port-

able programs, that is, programs that can be run without change on a variety of

hardware. The standard makes portability issues explicit, and prescribes a set of

constants that characterize the machine on which the program is run.

C is not a strongly-typed language, but as it has evolved, its type-checking

has been strengthened. The original definition of C frowned on, but permitted,

the interchange of pointers and integers; this has long since been eliminated, and

the standard now requires the proper declarations and explicit conversions that

had already been enforced by good compilers. The new function declarations are

another step in this direction. Compilers will warn of most type errors, and there

is no automatic conversion of incompatible data types. Nevertheless, C retains

the basic philosophy that programmers know what they are doing; it only

requires that they state their intentions explicitly.

C, like any other language, has its blemishes. Some of the operators have the

wrong precedence; some parts of the syntax could be better. Nonetheless, C has

proven to be an extremely effective and expressive language for a wide variety

of programming applications.

The book is organized as follows. Chapter 1 is a tutorial on the central part of

C. The purpose is to get the reader started as quickly as possible, since we

believe strongly that the way to learn a new language is to write programs in it.

The tutorial does assume a working knowledge of the basic elements of pro-

gramming; there is no explanation of computers, of compilation, nor of the

meaning of an expression like n=n+ 1. Although we have tried where possible to

show useful programming techniques, the book is not intended to be a reference

work on data structures and algorithms; when forced to make a choice, we have

concentrated on the language.

4 INTRODUCTION

Chapters 2 through 6 discuss various aspects of C in more detail, and rather

more formally, than does Chapter 1, although the emphasis is still on examples

of complete programs, rather than isolated fragments. Chapter 2 deals with the

basic data types, operators and expressions. Chapter 3 treats control flow: if-else,

switch, while, for, etc. Chapter 4 covers functions and program structure-external

variables, scope rules, multiple source files, and so on-and also touches on the

preprocessor. Chapter 5 discusses pointers and address arithmetic. Chapter 6 covers

structures and unions.

Chapter 7 describes the standard library, which provides a common interface

to the operating system. This library is defined by the ANSI standard and is

meant to be supported on all machines that support C, so programs that use it for

input, output, and other operating system access can be moved from one sys-tem

to another without change.

Chapter 8 describes an interface between C programs and the UNIX operat-

ing system, concentrating on input/output, the file system, and storage alloca-

tion. Although some of this chapter is specific to UNIX systems, programmers

who use other systems should still find useful material here, including some

insight into how one version of the standard library is implemented, and sugges-

tions on portability.

Appendix A contains a language reference manual. The official statement of

the syntax and semantics of C is the ANSI standard itself. That document,

however, is intended foremost for compiler writers. The reference manual here

conveys the definition of the language more concisely and without the same

legalistic style. Appendix B is a summary of the standard library, again for users

rather than implementers. Appendix C is a short summary of changes from the

original language. In cases of doubt, however, the standard and one's own

compiler remain the final authorities on the language.

cHAPTER 1: A Tutorial Introduction

Let us begin with a quick introduction to C. Our aim is to show the essen-tial

elements of the language in real programs, but without getting bogged down in

details, rules, and exceptions. At this point, we are not trying to be complete or even

precise (save that the examples are meant to be correct). We want to get you as

quickly as possible to the point where you can write useful programs, and to do that

we have to concentrate on the basics: variables and constants, arithmetic, control

flow, functions, and the rudiments of input and output. We are intentionally leaving

out of this chapter features of C that are important for writing bigger programs.

These include pointers, structures, most of C's rich set of operators, several control-

flow statements, and the standard library.

This approach has its drawbacks. Most notable is that the complete story on any

particular language feature is not found here, and the tutorial, by being brief, may

also be misleading. And because the examples do not use the full power of C, they

are not as concise and elegant as they might be. We have tried to minimize these

effects, but be warned. Another drawback is that later chapters will necessarily

repeat some of this chapter. We hope that the repeti-tion will help you more than it

annoys.

In any case, experienced programmers should be able to extrapolate from the

material in this chapter to their own programming needs. Beginners should sup-

plement it by writing small, similar programs of their own. Both groups can use it

as a framework on which to hang the more detailed descriptions that begin in

Chapter 2.

1. 1 Getting Started

The only way to learn a new programming language is by writing programs in

it. The first program to write is the same for all languages:

Print the words

hello, world

This is the big hurdle; to leap over it you have to be able to create the program

6 A TUTORIAL INTRODUCTION CHAPTER 1

text somewhere, compile it successfully, load it, run it, and find out where your

output went. With these mechanical details mastered, everything else is com-

paratively easy.

In C, the program to print "hello, world" is

#include <stdio.h>

main()

{

printf("hello, world\n");

}

Just how to run this program depends on the system you are using. As a

specific example, on the UNIX operating system you must create the program in

a file whose name ends in ". c", such as hello. c, then compile it with the

command

cc hello.c

If you haven't botched anything, such as omitting a character or misspelling

something, the compilation will proceed silently, and make an executable file called

a. out. If you run a. out by typing the command

a.out

it will print

hello, world

On other systems, the rules will be different; check with a local expert.

Now for some explanations about the program itself. A C program, what-

ever its size, consists of functions and variables. A function contains state-ments

that specify the computing operations to be done, and variables store values

used during the computation. C functions are like the subroutines and functions

of Fortran or the procedures and functions of Pascal. Our example is a function

named, main. Normally you are at liberty to give functions whatever names you

like, but "main" is special-your program begins executing at the beginning of

main. This means that every program must have a main some-where.

main will usually call other functions to help perform its job, some that you

wrote, and others from libraries that are provided for you. The first line of the

program,

#include <stdio.h>

tells the compiler to include information about the standard input/output library;

this line appears at the beginning of many C source files. The standard library is

described in Chapter 7 and Appendix B.

One method of communicating data between functions is for the calling

function to provide a list of values, called arguments, to the function it calls.

The parentheses after the function name surround the argument list. In this

SECTION 1.1 GETTING STARTED 7

#include <stdio.h>

include information about standard library

main()

define a function named main

that receives no argument values

{

statements of main are enclosed in braces

print£( "hello, world\n");

main calls library function print£ to

print this sequence of characters;

}

\n represents the newline character

The first C program.

example, main is defined to be a function that expects no arguments, which is

indicated by the empty list ( ) .

The statements of a function are enclosed in braces { }. The function main

contains only one statement,

print£( "hello, world\n");

A function is called by naming it, followed by a parenthesized list of arguments, so

this calls the function printf with the argument "hello, world\n". printf is a library

function that prints output, in this case the string of char-acters between the quotes.

A sequence of characters in double quotes, like "hello, world\n ", is called a

character string or string constant. For the moment our only use of character strings

will be as arguments for printf and other functions.

The sequence \n in the string is C notation for the newline character, which

when printed advances the output to the left margin on the next line. If you leave out

the \n (a worthwhile experiment), you will find that there is no line advance after the

output is printed. You must use \n to include a newline character in the printf

argument; if you try something like

printf("hello, world

II ) ;

the C compiler will produce an error message.

printf never supplies a newline automatically, so several calls may be used to

build up an output line in stages. Our -first program could just as well have been

written

8 A TUTORIAL INTRODUCTION CHAPTER 1

#include <stdio.h>

main()

{

print£ ( "hello, " ) ;

print£ ("world") ;

print£ ( "'\n") ;

}

to produce identical output.

Notice that '\n represents only a single character. An escape sequence like '\n

provides a general and extensible mechanism for representing hard-to-type or

invisible characters. Among the others that C provides are '\t for tab, '\b for

backspace, '\" for the double quote, and '\'\ for the backslash itself. There is a

complete list in Section 2.3.

Exercise 1-1. Run the "hello, world" program on your system. Experiment with

leaving out parts of the program, to see what error messages you get. 0

Exercise 1-2. Experiment to find out what happens when printf's argument string

contains '\c, where c is some character not listed above. 0

1.2 Variables and Arithmetic Expressions

The next program uses the formula • C- (5/9)(" F-32) to print the follow-ing

table of Fahrenheit temperatures and their centigrade or Celsius equivalents:

0 -17

20 -6

40 4

60 15

80 26

100 37

120 48

140 60

160 71

180 82

200 93

220 104

240 115

260 126

280 137

300 148

The program itself still consists of the det:inition of a single function named

main. It is longer than the one that printed "hello, world", but not compli-cated. It

introduces several new ideas, including comments, declarations, vari-ables,

arithmetic expressions, loops, and formatted output.

SECTION 1.2 VARIABLES AND ARITHMETIC EXPRESSIONS 9

#include <stdio.h>

I* print Fahrenheit-Celsius table

for fahr = 0, 20, ... , 300 */

main()

{

int fahr, celsius;

int lower, upper, step;

lower = 0;

upper = 300;

lower limit of

upper limit */

temperature table

fahr = lower;

while (fahr <= upper) {

celsius = 5 * (fahr-32) I 9;

printf("%d\t%d\n", fahr, celsius);

fahr = fahr + step;

}

The two lines

/* print Fahrenheit-Celsius table for fahr =

o, 20, ... , 300 *I

are a comment, which in this case explains briefly what the program does. Any

characters between / * and */ are ignored by the compiler; they may be used

freely to make a program easier to understand. Comments may appear any-where

a blank or tab or newline can.

In C, all variables must be declared before they are used, usually at the

beginning of the function before any executable statements. A declaration

announces the properties of variables; it consists of a type name and a list of

variables, such as

int fahr, celsius;

int lower, upper, step;

The type int means that the variables listed are integers, by contrast with float,

which means floating point, i.e., numbers that may have a fractional part. The

range of both int and float depends on the machine you are using; 16-bit ints,

which lie between -32768 and +32767, are common, as are 32-bit ints. A float

number is typically a 32-bit quantity, with at least six significant digits and

magnitude generally between about 10-

and 10+

•

C provides several other basic data types besides int and float, including:

char

short

long

double

character-a single byte

short integer

long integer

double-precision floating point

10 A TUTORIAL INTRODUCTION CHAPTER 1

The sizes of these objects are also machine-dependent. There are also arrays,

structures and unions of these basic types, pointers to them, and functions that

return them, all of which we will meet in due course.

Computation in the temperature conversion program begins with the assign-

ment statements

lower = 0;

upper = 300;

step = 20;

fahr = lower;

which set the variables to their initial values. Individual statements are ter-

minated by semicolons.

Each line of the table is computed the same way, so we use a loop that

repeats once per output line; this is the purpose of the while loop

while (fahr <= upper) {

}

The while loop operates as follows: The condition in parentheses is tested. If it is

true (fahr is less than or equal to upper), the body of the loop (the three statements

enclosed in braces) is executed. Then the condition is re-tested, and if true, the

body is executed again. When the test becomes false (fahr exceeds upper) the

loop ends, and execution continues at the statement that follows the loop. There

are no further statements in this program, so it terminates.

The body of a while can be one or more statements enclosed in braces, as in

the temperature converter, or a single statement without braces, as in

while (i < j)

i = 2 * i;

In either case, we will always indent the statements controlled by the while by

one tab stop (which we have shown as four spaces) so you can see at a glance

which statements are inside the loop. The indentation emphasizes the logical

structure of the program. Although C compilers do not care about how a pro-

gram looks, proper indentation and spacing are critical in making programs easy

for people to read. We recommend writing only one statement per line, and using

blanks around operators to clarify grouping. The position of braces is less

important, although people hold passionate beliefs. We have chosen one of

several popular styles. Pick a style that suits you, then use it consistently.

Most of the work gets done in the body of the loop. The Celsius tempera-ture

is computed and assigned to the variable celsius by the statement

celsius = 5 * (fahr-32) I 9;

The reason for multiplying by 5 and then dividing by 9 instead of just multiply-

ing by 5/9 is that in C, as in many other languages, integer division truncates:

any fractional part is discarded. Since 5 and 9 are integers, 5/9 would be

truncated to zero and so all the Celsius temperatures would be reported as zero.

SECTION 1.2 VARIABLES AND ARITHMETIC EXPRESSIONS 11

This example also shows a bit more of how print£ works. print£ is a general-

purpose output formatting function, which we will describe in detail in Chapter

7. Its first argument is a string of characters to be printed, with each

% indicating where one of the other (second, third, .. .) arguments is to be substi-

tuted, and in what form it is to be printed. For instance, %d specifies an integer

argument, so the statement

printf("%d\t%d\n", fahr, celsius);

causes the values of the two integers fahr and celsius to be printed, with a tab (\t)

between them.

Each % construction in the first argument of print£ is paired with the

corresponding second argument, third argument, etc.; they must match up prop-

erly by number and type, or you'll get wrong answers.

By the way, print£ is not part of the C language; there is no input or out-put

defined in C itself. prihtf is just a useful function from the standard library of

functions that are normally accessible to C programs. The behavior of print£ is

defined in the ANSI standard, however, so its properties should be the same with

any compiler and library that conforms to the standard.

In order to concentrate on C itself, we won't talk much about input and out-

put until Chapter 7. In particular, we will defer formatted input until then. If you

have to input numbers, read the discussion of the function scan£ in Sec-tion 7 .4.

scan£ is like print£, except that it reads input instead of writing output.

There are a couple of problems with the temperature conversion program.

The simpler one is that the output isn't very pretty because the numbers are rtot

right-justified. That's easy to fix; if we augment each %d in the print£ state-ment

with a width, the numbers printed will be right-justified in their fields. For

instance, we might say

printf(""3d "6d\n", fahr, celsius);

to print the first number of each line in a field three digits wide, and the second

in a field six digits wide, like this:

0 -17

20 -6

40 4

60 15

80 26

100 37

The more serious problem is that because we have used integer arithmetic,

the Celsius temperatures are not very accurate; for instance, 0 • F is actually

about -11.s·c, not -17. To get more accurate answers, we should use floating-

point arithmetic instead of integer. This requires some changes in the program.

Here is a second version:

12 A TUTORIAL INTRODUCTION CHAPTER l

#include <stdio.h>

I* print Fahrenheit-Celsius table

for fahr = 0, 20, ... , 300; floating-point version*/ main()

{

float fahr, celsius;

int lower, upper, step;

lower = 0;

upper = 300;

step = 20;

/* lower limit of temperature table */ I* upper limit

/* step size */

fahr = lower;

while (fahr <= upper) {

celsius = (5.0/9.0) * (fahr-32.0); printf("%3.0f

%6.1f\n", fahr, celsius); fahr = fahr + step;

}

This is much the same as before, except that fahr and celsius are declared to be

float, and the formula for conversion is written in a more natural way. We were

unable to use 5/9 in the previous version because integer division would truncate it

to zero. A decimal point in a constant indi-cates that it is floating point, however, so

5. 0/9. 0 is not truncated because it is the ratio of two floating-point values.

If an arithmetic operator has integer operands, an integer operation is per-

formed. If an arithmetic operator has one floating-point operand and one integer

operand, however, the integer will be converted to floating point before the

operation is done. If we had written fahr-32, the 32 would be automati-cally

converted to floating point. Nevertheless, writing floating-point constants with

explicit decimal points even when they have integral values emphasizes their

floating-point nature for human rea9ers.

The detailed rules for when integers are converted to floating point are in

Chapter 2. For now, notice that the assignment

fahr = lower;

and the test

while (fahr <= upper)

also work in the natural way-the int is converted to float before the opera-tion is

done.

The printf conversion specification %3. Of says that a floating-point number

(here fahr) is to be printed at least three characters wide, with no decimal point and

no fraction digits. %6 . 1f describes another number (celsius) that is to be printed at

least six characters wide, with 1 digit after the decimal point. The output looks like

this:

SECTION 1.3 THE FOR STATEMENT 13

0 -17.8

20-6.7

404.4

Width and prec1s1on may be omitted from a specification: %6£ says that the

number is to be at least six characters wide; %. 2£ specifies two characters after

the decimal point, but the width is not constrained; and %£ merely says to print

the number as floating point.

print as decimal integer

%6d

print as decimal integer, at least

6 characters wide

%£

print as floating point

%6£

print as floating point, at least 6

characters wide

%. 2£print as floating point, 2 characters after decimal point

%6. 2£ print as floating point, at least 6 wide and 2 after decimal point

Among others, print£ also recognizes %o for octal, %x for hexadecimal, %c for

character, %s for character string, and %%for % itself.

Exercise l-3. Modify the temperature conversion program to print a heading

above the table. 0

Exercise l-4. Write a program to print the corresponding Celsius to Fahrenheit

table. o

1.3 The For Statement

There are plenty of different ways to write a program for a particular task.

Let's try a variation on the temperature converter.

#include <stdio.h>

/* print Fahrenheit-Celsius table *' main()

{

int fahr;

for (fahr = 0; fahr <= 300; fahr = fahr + 20) printf("%3d %6.1£\n",

fahr, (5.0/9.0)*(fahr-32));

}

This produces the same answers, but it certainly looks different. One major

change is the elimination of most of the variables; only fahr remains, and we

have made it an int. The lower and upper limits and the step size appear only as

constants in the for statement, itself a new construction, and the expression that

computes the Celsius temperature now appears as the third argument of print£

instead of as a separate assignment statement.

This last change is an instance of a general rule-in any context where it is

14 A TUTORIAL INTRODUCTION CHAPTER 1

permissible to use the value of a vari:.ible of some type, you can use a more com-

plicated eApression of that type. Since the third argument of printf must be a

floating-point value to match the %6. 1f, any floating-point expression can occur

there.

The for statement is a loop, a generalization of the while. If you compare it to the

earlio...r while, its operation should be clear. Within the parentheses, there are three

parts, separated by semicolons. The first part, the initialization

fahr = 0

is done once, before the loop proper is entered. The second part is the test or

condition that controls the loop:

fahr <= 300

This condition is evaluated; if it is true, the body of the loop (here a single printf) is

executed. Then the increment step

fahr = fahr + 20

is executed, and the condition re-evaluated. The loop termhates if the condition has

become false. As with the while, the body of the loop can be a single statement, or a

group of statements enclosed in braces. The initialization, con-dition, and increment

can be any expressions.

The choice between while and for is arbitrary, based on which seems clearer. The

fo;r is usually appropriate for loops 'in which the initialization aud increment are

single statements and logically related, since it is more compact than while and it

keeps the loop control statements together in one place.

Exercise 1-5. Modify the temperature conversion program to print the table in

reverse order, that is, from 300 degrees to 0. D

1.4 Symbolic Constants

A final observation before we leave temperature conversion forever. It's bad

practice to bury "magic numbers" like 300 and 20 in a program; they convey little

information to someone who might have to read the program later, and they are

hard to change In a systematic way. One way to deal with magic numbers is to give

them ~eaningful names. A #define line defines a sym-bolic name or symbolic constant

to be a particular string of characters:

#define name replacement text

Thereafter, any occurrence of name (not in quotes and not part of another name)

will be replaced by the corresponding replacement text. The name has the same

form as a variable name: a sequence of letters and digits that begins with a letter.

The replacement text can be any sequence of characters; it is not limited to

numbers.

SECTION I.S

CHARACTER INPUT AND OUTPUT

#include

<stdio.h>

#define

LOWER

/* lower

limit

of table */

#define

UPPER

300

upper

limit

#define

STEP

step

size */

I* pr-int Fahrenheit-Celsius table */ main()

{

int fahr;

for (fahr = LOWER; fahr <= UPPER; fahr = fahr + STEP)

printf("%3d %6.1f\n", fahr, (5.0/9.0)*(fahr-32));

}

The quantities LOWER, UPPER and STEP are symbolic constants, not variables, so

they do not appear in declarations. Symbolic constant names are convention-ally

written in upper case so they can be readily distinguished from lower case

variable names. Notice that there is no semicolon at the end of a #define line.

1.5 Character Input and Output

We are now going to consider a family of related programs for processing

character data. You will find that many programs are just expanded versions of

the prototypes that we discuss here.

The model of input and output supported by the standard library is very sim-

ple. Text input or output, regardless of where it originates or where it goes to, is

dealt with as streams of characters. A text stream is a sequence ofcharac-ters

divided into lines; each line consists of zero or more characters followed by a

newline character. It is the responsibility of the library to make each input or

output stream conform to this model; the C programmer using the library need

not worry about how lines are represented outside the program.

The standard library provides several functions for reading or writing one

character at a time, of which getchar and putchar are the simplest. Each time it is

called, getchar reads the next input character from a text stream and returns that

as its value. That is, after

c = getchar()

the variable c contains the next character of input. The characters normally come

from the keyboard; input from files is discussed in Chapter 7.

The function putchar prints a character each time it is called:

putchar(c)

prints the contents of the integer variable c as a character, usually on the screen.

Calls to putchar and print£ may be interleaved; the output will

16 A TUTORIAL INTRODUCTION CHAPTER l

appear in the order in which the calls are made.

1.5. 1 File Copying

Given getchar and putchar, you can write a surprising amount of useful code

without knowing anything more about input and output. The simplest example is

a program that copies its input to its output one character at a time:

read a character

while (character is not end -of-file indicator)

output the character just read

read a character

Converting this into C gives

#include <stdio.h>

I* copy input to output; 1st version */ main()

{

int c;

c = getchar();

while (c I= EOF) {

putchar(c);

c = getchar();

}

The relational operator I = means "not equal to."

What appears to be a character on the keyboard or screen is of course, like

everything else, stored internally just as a bit pattern. The type char is specifi-

cally meant for storing such character data, but any integer type can be used. We

used int for a subtle but important reason.

The problem is distinguishing the end of the input from valid data. The

solution is that getchar returns a distinctive value when there is no more input, a

value that cannot be confused with any real character. This value is called EOF,

for "end of file." We must declare c to be a type big enough to hold any value

that getchar returns. We can't use char since c must be big enough to hold EOF in

addition to any possible char. Therefore we use int.

EOF is an integer defined in <stdio. h>, but the specific numeric value doesn't

matter as long as it is not the same as any char value. By using the symbolic

constant, we are assured that nothing in the program depends on the specific

numeric value.

The program for copying would be written more concisely by experienced C

programmers. In C, any assignment, such as

c = getchar()

SECTION 1.5 CHARACTER INPUT AND OUTPUT

is an expression and has a value, which is the value of the left hand side after the

assignment. This means that an assignment can appear as part of a larger

expression. If the assignment of a character to c is put inside the test part of a

while loop, the copy program can be written this way:

#include <stdio.h>

I* copy input to output; 2nd version */ main()

{

int c;

while ((c = qetchar()) I= EOF)

putchar (c) ;

}

The while gets a character, assigns it to c, and then tests whether the charac-ter

was the end-of-file signal. If it was not, the body of the while is executed,

printing the character. The while then repeats. When the end of the input is

finally reached, the while terminates and so does main.

This version centralizes the input-there is now only one reference to getchar-

and shrinks the program. The resulting program is more compact, and, once the

idiom is mastered, easier to read. You'll see this style often. (It's possible to get

carried away and create impenetrable code, however, a tendency that we will try

to curb.)

The parentheses around the assignment within the condition are necessary.

The precedence of I= is higher than that of =, which means that in the absence of

parentheses the relational test I = would be done before the assignment =. So the

statement

c = qetchar() I= EOF

is equivalent to

c = (qetchar() I= EOF)

This has the undesired effect of setting c to 0 or 1, depending on whether or not

the call of getchar encountered end of file. (More on this in Chapter 2.)

Exercise 1-6. Verify that the expression getchar ( ) I= EOF is 0 or 1. 0

Exercise 1-7. Write a program to print the value of EOF. 0

1.5.2 Character Counting

The next program counts characters; it is similar to the copy program.

18A TUTORIAL INTRODUCTION

CHAPTER I

#include

<stdio.h>

/* count

characters

input; 1st version */

main()

{

long

nc;

nc = 0;

EOF)

while

(getchar()

++nc;

printf ( 11 %ld\n 11 ,

nc) ;

}

The statement

++nc;

presents a new operator, ++, which means increment by one. You could instead write nc

= nc+ 1 but ++nc is more concise and often more efficient. There is a corresponding

operator -- to decrement by 1. The operators ++ and --can be either prefix operators

(++nc) or postfix (nc++); these two forms have dif-ferent values in expressions, as will

be shown in Chapter 2, but ++nc and nc++ both increment nc. For the moment we will

stick to the prefix form.

The character counting program accumulates its count in a long variable instead of

an int. long integers are at least 32 bits. Although on some machines, int and long are the

same size, on others an int is 16 bits, with a maximum value of 32767, and it would take

relatively little input to overflow an int counter. The conversion specification %ld tells

print£ that the corresponding argument is a long integer.

It may be possible to cope with even bigger numbers by using a double

(double precision float).

will

also

use a

for statement instead of a

while, to illustrate another way to write the loop.

#include <stdio.h>

I* count characters in input; 2nd version */

main()

{

double

nc;

for (nc

= 0;

getchar()

EOF;

++nc)

nc);

printf ( 11 %. Of\n 11 ,

}

print£ uses %£ for both float and double; %. Of suppresses printing of the decimal point

and the fraction part, which is zero.

The body of this for loop is empty, because all of the work is done in the test and

increment parts. But the grammatical rules of C require that a for statement have a body.

The isolated semicolon, called a null statement, is there

SECTION 1.5 CHARACTER INPUT AND OUTPUT 19

to satisfy that requirement. We put it on a separate line to make it visible. Before we

leave the character counting program, observe that if the input

contains no characters, the while or for test fails on the very first call to getchar, and

the program produces zero, the right answer. This is important. One of the nice

things about while and for is that they test at the top of the loop, before proceeding

with the body. If there is nothing to do, nothing is done, even if that means never

going through the loop body. Programs should act intelligently when given zero-

length input. The while and for statements help ensure that programs do reasonable

things with boundary conditions.

1.5.3 Line Counting

The next program counts input lines. As we mentioned above, the standard

library ensures that an input text stream appears as a sequence of lines, each

terminated by a newline. Hence, counting lines is just counting newlines:

#include <stdio.h>

/* count lines in input */

main()

{

int c, nl;

nl = 0;

while ((c = getchar()) I= EOF)

if (c == '\.n' >

++nl;

printf ( ""d\.n", nl) ;

}

The body of the while now consists of an if, which in turn controls the increment

++nl. The if statement tests the parenthesized condition, and if the condition is true,

executes the statement (or group of statements in braces) that follows. We have

again indented to show what is controlled by what.

The double equals sign == is the C notation for "is equal to" {like Pascal's single

= or Fortran's . EQ.). This symbol is used to distinguish the equality test from the

single = that C uses for assignment. A word of caution: newcomers to C

occasionally write = when they mean ==. As we will see in Chapter 2, the result is

usually a legal expression, so you will get no warning.

A character written between single quotes represents an integer value equal to

the numerical value of the character in the machine's character set. This is called a

character constant, although it is just another way to write a small integer. So, for

example, 'A' is a character constant; in the ASCII character set its value is 65, the

internal representation of the character A. Of course 'A' is to be preferred over 65: its

meaning is obvious, and it is independent of a par-ticular character set.

The escape sequences used in string constants are also legal in character

20 A TUTORIAL INTRODUCTION CHAPTER 1

constants, so '\n'stands for the value of the newline character, which is 10 in

ASCII. You should note carefully that '\n'is a single character, and in expressions

is just an integer; on the other hand, 11 \n 11 is a string constant that happens to

contain only one character. The topic of strings versus characters is discussed

further in Chapter 2.

Exercise 1-8. Write a program to count blanks, tabs, and newlines. D

Exercise 1-9. Write a program to copy its input to its output, replacing each

string of one or more blanks by a single blank. 0

Exercise 1-10. Write a program to copy its input to its output, replacing each tab

by \t, each backspace by \b, and each backslash by \\. This makes tabs and

backspaces visible in an unambiguous way. o

1.5.4 Word Counting

The fourth in our series of useful programs counts lines, words, and charac-

ters, with the loose definition that a word is any sequence of characters that does

not contain a blank, tab or newline. This is a bare-bones version of the UNIX

program we.

#include <stdio.h>

#define IN 1 /* inside a word */

#define OUT 0 /* outside a word */

f* count lines, words, and characters in input */ main()

{

int c, nl, nw, nc, state;

state = OUT;

nl = nw = nc = 0;

while ((c = getchar()) I= EOF) {

++nc;

if ( c == '\n' )

++nl;

if (c ==''IIc== '\n' II c == '\t')

state = OUT;

else if (state == OUT) {

state = IN;

++nw;

}

printf ( "%d %d %d\n" , nl, nw, nc) ;

}

Every time the program encounters the first character of a word, it counts

SECTION 1.5 CHARACTER INPUT AND OUTPUT 21

one more word. The variable state records whether the program is currently in a

word or not; initially it is "not in a word," which is assigned the value OUT. We

prefer the symbolic constants IN and OUT to the literal values 1 and 0 because

they make the program more readable. In a program as tiny as this, it makes little

difference, but in larger programs, the increase in clarity is well worth the

modest extra effort to write it this way from the beginning. You'll also find that

it's easier to make extensive changes in programs where magic numbers appear

only as symbolic constants.

The line

nl = nw = nc = 0;

sets all three variables to zero. This is not a special case, but a consequence of

the fact that an assignment is an expression with a value and assignments asso-

ciate from right to left. It's as if we had written

(nw

(nc

= 0));

The operator

: : means OR, so the line

== ' ' ::

c ==

'\n'

'\t')

says "if c is a blank or c is a n~wline or c is a tab". (Recall that the escape

sequence \t is a visible representation of the tab character.) There is a

corresponding operator && for AND; its precedence is just higher than : :.

Expressions connected by && or : : are evaluated left to right, and it is guaranteed

that evaluation will stop as soon as the truth or falsehood is known. If c is a blank,

there is no need to test whether it is a newline or tab, so these tests are not made.

This isn't particularly important here, but is significant in more complicated

situations, as we will soon see.

The example also shows an else, which specifies an alternative action if the

condition part of an if statement is false. The general form is

if (expression)

statement

else

statement

One and only one of the two statements associated with an if-else is per-formed.

If the expression is true, statement

is executed; if not, statement

is executed.

Each statement can be a single statement or several in braces. In the word count

program, the one after the else is an if that controls two state-ments in braces.

Exercise 1-11. How would you test the word count program? What kinds of input

are most likely to uncover bugs if there are any? 0

Exercise 1-12. Write a program that prints its input one word per line. o

22 A TUTORIAL INTRODUCTION CHAPTER I

1.6 Arrays

Let us write a program to count the number of occurrences of each digit, of

white space characters (blank, tab, newline), and of all other characters. This is

artificial, but it permits us to illustrate several aspects of C in one program.

There are twelve categories of input, so it is convenient to use an array to

hold the number of occurrences of each digit, rather than ten individual vari-

ables. Here is one version of the program:

#include <stdio.h~

/* count digits, white space, others */ main()

{

int c, i, nwhite, nother;

int ndigit[10];

nwhite = nother = 0;

for (i = 0; i < 10; ++i)

ndigit[i] = 0;

while ((c = getchar()) I= EOF)

if (C >= '0' && C <= '9')

++ndigit[c-'0'];

else if (c =='' :: c == '\n' :: c == '\t')

++nwhite;

~lse

++nother;

printf("digits =");

for (i = 0; i < 10; ++i)

print£(" %d", ndigit[i]);

print£(", white space = %d, other = %d\n", nwhi te , nother ) ;

}

The output of this program on itself is

digits = 9 3 0 0 0 0 0 0 0 1, white space = 123, other = 345

The declaration

int ndigit[10];

declares ndigi t to be an array of 10 integers. Array subscripts always start at zero

in C, so the elements are ndigi t [ 0 1, ndigi t [ 1 1, ... , ndigi t [ 9 1. This is reflected

in the for loops that initialize and print the array.

A subscript can be any integer expression, which includes integer variables

like i, and integer constants.

This particular program relies on the properties of the character representa-

tion of the digits. For example, the test

SECTION 1.6

ARRAYS

if (c >= '0' && c <=

'9') ...

determines whether the character in c is a digit. If it is, the numeric value of that

digit is

c - '0,

This works only if '0 ', ' 1 ', ... , '9 ' have consecutive increasing values. For-tunately,

this is true for all character sets.

By definition, chars are just small integers, so char variables and constants are

identical to ints in arithmetic expressions. This is natural and convenient; for

example, c-' 0' is an integer expression with a value between 0 and 9

corresponding to the character '0' to '9' stored in c, and is thus a valid sub-script

for the array ndigi t.

The decision as to whether a character is a digit, white space, or something

else is made with the sequence

if (c >= '0' && c <= '9')

++ndigit[c-'0'];

else if (c ==''ll c == '\n' ll c -- '\t')

++nwhite;

else

++nother;

The pattern

if (condition

)

statement,

else if (condition

)

statement 2

else

statement,

occurs frequently in programs as a way to express a multi-way decision. The

conditions are evaluated in order from the top until some condition is satisfied; at

that point the corresponding statement part is executed, and the entire con-struction

is finished. (Any statement can be several statements enclosed in braces.) If none of

the conditions is satisfied, the statement after the final else is executed if it is present.

If the final else and statement are omitted, as in the word count program, no action

takes place. There can be any number of

else if (condition)

statement

groups between the initial if and the final else.

As a matter of style, it is advisable to format this construction as we have

shown; if each if were indented past the previous else, a long sequence of

decisions would march off the right side of the page.

24 A TUTORIAL INTRODUCTION CHAPTER I

The switch statement, to be discussed in Chapter 3, provides another way to

write a multi-way branch that is particularly suitable when the condition is

whether some integer or character expression matches one of a set of constants.

For contrast, we will present a switch version of this program in Section 3.4.

Exercise l-13. Write a program to print a histogram of the lengths of words in its

input. It is easy to draw the histogram with the bars horizontal; a vertical

orientation is more challenging. 0

Exercise l-14. Write a program to print a histogram of the frequencies of dif-

ferent characters in its input. o

1.7 Functions

In C, a function is equivalent to a subroutine or function in Fortran, or a

procedure or function in Pascal. A function provides a convenient way to

encapsulate some computation, which can then be used without worrying about

its implementation. With properly designed functions, it is possible to ignore how

a job is done; knowing what is done is sufficient. C makes the use of func-tions

easy, convenient and efficient; you will often see a short function defined and called

only once, just because it clarifies some piece of code.

So far we have used only functions like print£, getchar, and putchar that have

been provided for us; now it's time to write a few of our own. Since C has no

exponentiation operator like the ** of Fortran, let us illustrate the mechanics of

function definition by writing a function power ( m, n) to raise an integer m to a

positive integer power n. That is, the value of power ( 2 , 5 ) is

32. This function is not a practical exponentiation routine, since it handles only

positive powers of small integers, but it's good enough for illustration. (The

standard library contains a function pow ( x, y) that computes xY.)

Here is the function power and a main program to exercise it, so you can see

the whole structure at once.

#include <stdio.h>

int power(int m, int n);

I• test power function •I

main()

{

int i;

for (i = 0; i < 10; ++i)

printf ( "%d "d %d\n", i, power( 2, i), power( -3, i)); return 0;

}

SECTION 1.7

FUNCTIONS

int

power: raise base to n-th power; n power(int base,

int n)

{

p =

for

n; ++i)

p = p

base;

return p;

}

A function definition has this form:

return-type function-name (parameter declarations, if any)

{

declarations

statements

}

Function definitions can appear in any order, and in one source file or several,

although no function can be split between files. If the source program appears in

several files, you may have to say more to compile and load it than if it all

appears in one, but that is an operating system matter, not a language attribute.

For the moment, we will assume that both functions are in the same file, so

whatever you have learned about running C programs will still work.

The function power is called twice by main, in the line

printf("%d %d %d\n", i, power(2,i), power(-3,i));

Each call passes two arguments to power, which each time returns an integer to

be formatted and printed. In an expression, power ( 2 , i ) is an integer just as 2

and i are. (Not all functions produce an integer value; we will take this up in

Chapter 4.)

The first line of power itself,

int power(int base, int n)

declares the parameter types and names, and the type of the result that the

function returns. The names used by power for its parameters are local to power,

and are not visible to any other function: other routines can use the same names

without conflict. This is also true of the variables i and p: the i in power is unrelated

to the i in main.

We will generally use parameter for a variable named in the parenthesized

list in a function definition, and argument for the value used in a call of the

function. The terms formal argument and actual argument are sometimes used for

the same distinction.

The value that power computes is returned to main by the return state-ment.

Any expression may follow return:

return expression ;

26 A TUTORIAL INTRODUCTION CHAPTER I

A function need not return a value; a return statement with no expression causes

control, but no useful value, to be returned to the caller, as does "falling off the

end" of a function by reaching the terminating right brace. And the cal-ling

function can ignore a value returned by a function.

You may have noticed that there is a return statement at the end of main.

Since main is a function like any other, it may return a value to its caller, which

is in effect the environment in which the program was executed. Typi-cally, a

return value of zero implies normal termination; non-zero values signal unusual

or erroneous termination conditions. In the interests of simplicity, we have

omitted return statements from our main functions up to this point, but we will

include them hereafter, as a reminder that programs should return status to their

environment.

The declaration

int power(int m, int n);

just before main says that power is a function that expects two int arguments and

returns an int. This declaration, which is called a function prototype, has to agree

with the definition and uses of power. It is an error if the definition of a function

or any uses of it do not agree with its prototype.

Parameter names need not agree. Indeed, parameter names are optional in a

function prototype, so for the prototype we could have written

int power(int, int);

Well-chosen names are good documentation, however, so we will often use them.

A note of history: The biggest change between ANSI C and earlier versions is

how functions are declared and defined. In the original definition of C, the power

function would have been written like this:

I* power: raise base to n-th power; n >= 0 */

I* (old-style version) */ power(base, n)

int base, n;

{

int i, p;

p = 1;

for (i = 1; i <= n; ++i)

p = p * base;

return p;

}

The parameters are named between the parentheses, and their types are declared

before the opening left brace; undeclared parameters are taken as int. (The body

of the function is the same as before.)

The declaration of power at the beginning of the program would have looked

like this:

SECTION 1.8 ARGUMENTS-CALL BY VALUE 27

int power();

No parameter list was permitted, so the compiler could not readily check that

power was being called correctly. Indeed, since by default power would have been

assumed to return an int, the entire declaration might well have been omitted.

The new syntax of function prototypes makes it much easier for a compiler to

detect errors in the number of arguments or their types. The old style of

declaration and definition still works in ANSI C, at least for a transition period,

but we strongly recommend that you use the new form when you have a com-

piler that supports it.

Exercise 1-15. Rewrite the temperature conversion program of Section 1.2 to use

a function for conversion. D

1.8 Arguments-Call by Value

One aspect of C functions may be unfamiliar to programmers who are used to

some other languages, particularly Fortran. In C, all function arguments are

passed "by value." This means that the called function is given the values of its

arguments in temporary variables rather than the originals. This leads to some

different properties than are seen with "call by reference" languages like For-tran

or with var parameters in Pascal, in which the called routine has access to the

original argument, not a local copy.

The main distinction is that in C the called function cannot directly alter a

variable in the calling function; it can only alter its private, temporary copy.

Call by value is an asset, however, not a liability. It usually leads to more

compact programs with fewer extraneous variables, because parameters can be

treated as conveniently initialized local variables in the called routine. For

example, here is a version of power that makes use of this property.

I• power: raise base to n-th power; n>=O; version 2 •I int power(int base, int

{

int p;

for (p = 1; n > 0; --n)

p = p * base;

return p;

}

The parameter n is used as a temporary variable, and is counted down (a for loop

that runs backwards) until it becomes zero; there is no longer a need for the

variable i. Whatever is done to n inside power has no effect on the argu-ment that

power was or~ginally called with.

When necessary, it is possible to arrange for a function to modify a variable

28 A TUTORIAL INTRODUCTION CHAPTER I

in a calling routine. The caller must provide the address of the variable to be set

(technically a pointer to the variable), and the called function must declare the

parameter to be a pointer and access the variable indirectly through it. We will

cover pointers in Chapter 5.

The story is different for arrays. When the name of an array is used as an

argument, the value passed to the function is the location or address of the

beginning of the array-there is no copying of array elements. By subscripting

this value, the function can access and alter any element of the array. This is the

topic of the next section.

1.9 Character Arrays

The most common type of array in C is the array of characters. To illus-trate

the use of character arrays and functions to manipulate them, let's write a

program that reads a set of text lines and prints the longest. The outline is sim-

ple enough:

while (there's another line)

if (it's longer than the previous longest)

save it

save its length

print longest line

This outline makes it clear that the program divides naturally into pieces. One

piece gets a new line, another tests it, another saves it, and the rest controls the

process.

Since things divide so nicely, it would be well to write them that way too.

Accordingly, let us first write a separate function getline to fetch the next line of

input. We will try to make the function useful in other contexts. At the minimum,

getline has to return a signal about possible end of file; a more useful design

would be to return the length of the line, or zero if end of file is encountered.

Zerois an acceptable end-of-file return because it is never a valid line length.

Every text line has at least one character; even a line containing only a newli'ne

has length 1.

When we find a line that is longer than the previous longest line, it must be

saved somewhere. This suggests a second function, copy, to copy the new line to

a safe place.

Finally, we need a main program to control getline and copy. Here is the

result.

SECTION 1.9 CHARACTER ARRAYS 29

#include <stdio.h>

#define MAXLINE 1000 /* maximum input

line size */

int getline(char line[], int maxline); void copy(char

to[], char from[]);

/* print longest input line */

main()

{

int len; /* current line length */

int max; /* maximum length seen so

char line[MAXLINE]; /* current input line */

char longest[MAXLINE]; /* longest line saved here */

max = 0;

while ( (len = getline (line, MAXLINE) ) > 0)

if ( len > max) {

max = len;

copy(longest, line);

}

if (max > 0) /* there was a line */ printf ( ""s" ,

longest) ;

return 0;

}

/* getline: read a line into s, return length */ int getline(char s[], int

lim)

{

int c, i;

for (i=O; i<lim-1 && (c=getchar())I=EOF && cl='\n'; ++i) s[i] = c;

if (c == '\n'){

s[i] = c;

++i;

}

s(i] = '\0';

return i;

}

/*copy: copy 'from' into 'to'; assume to is big enough*/ void copy(char to[], char

from[])

{

int i;

i = 0;

while ( (to[i] = from(i]) I= '\0')

++ij

}

30 A TUTORIAL INTRODUCTION CHAPTER I

The functions getline and copy are declared at the beginning of the pro-gram,

which we assume is contained in one file.

main and getline communicate through a pair of arguments and a returned value.

In getline, the arguments are declared by the line

int getline(char s[], int lim)

which specifies that the first argument, s, is an array, and the second, lim, is an

integer. The purpose of supplying the size of an array in a declaration is to set

aside storage. The length of the array s is not necessary in getline since its size is

set in main. getline uses return to send a value back to the caller, just as the

function power did. This line also declares that getline returns an int; since int is

the default return type, it could be omitted.

Some funCtions return a useful value; others, like copy, are used only for

their effect and return no value. The return type of copy is void, which states

explicitly that no value is returned.

getline puts the character '\0' (the null character, whose value is zero) at the end

of the array it is creating, to mark the end of the string of characters. This

convention is also used by the C language: when a string constant like

"hello\n"

appears in a C program, it is stored as an array of characters containing the

characters of the string and terminated with a '\0' to mark the end.

I h I e I 1 I 1 I o I \n I 'o I

The %s format specification in print£ expects the corresponding argument to be a

string represented in this form. copy also relies on the fact that its input argument

is terminated by '\0 ', and it copies this character into the output argument. (All

of this implies that '\0' is not a part of normal text.)

It is worth mentioning in passing that even a program as small as this one

presents some sticky design problems. For example, what should main do if it

encounters a line which is bigger than its limit? getline works safely, in that it stops

collecting when the array is full, even if no newline has been seen. By testing the

length and the last character returned, main can determine whether the line was too

long, and then cope as it wishes. In the interests of brevity, we have ignored the

issue.

There is no way for a user of getline to know in advance how long an input

line might be, so getline checks for overflow. On the other hand, the user of copy

already knows (or can find out) how big the strings are, so we have chosen not to

add error checking to it.

Exercise 1-16. Revise the main routine of the longest-line program so it will

correctly print the length of arbitrarily long input lines, and as much as possible

of the text. D

SECTION 1.10 EXTERNAL VARIABLES AND SCOPE 31

Exercise 1-17. Write a program to print all input lines that are longer than 80

characters. 0

Exercise 1-18. Write a program to remove trailing blanks and tabs from each

line of input, and to delete entirely blank lines. o

Exercise 1-19. Write a function reverse ( s} that reverses the character string s.

Use it to write a program that reverses its input a line at a time. 0

1.10 External Variables and Scope

The variables in main, such as line, longest, etc., are private or local to main.

Because they are declared within main, no other function can have direct access to

them. The same is true of the variables in other functions; for example, the variable i

in getline is unrelated to the i in copy. Each local variable in a function comes into

existence only when the function is called, and disappears when the function is

exited. This is why such variables are usually known as automatic variables,

following terminology in other languages. We will use the term automatic

henceforth to refer to these local variables. (Chapter 4 discusses the static storage

class, in which local variables do retain their values between calls.)

Because automatic variables come and go with function invocation, they do

not retain their values from one call to the next, and must be explicitly set upon

each entry. If they are not set, they will contain garbage.

As an alternative to automatic variables, it is possible to define variables that

are external to all functions, that is, variables that can be accessed by name by

any function. (This mechanism is rather like Fortran COMMON or Pascal vari-

ables declared in the outermost block.) Because external variables are globally

accessible, they can be used instead of argument lists to communicate data

between functions. Furthermore, because external variables remain in existence

permanently, rather than appearing and disappearing as functions are called and

exited, they retain their values even after the functions that set them have

returned.

An external variable must be defined, exactly once, outside of any function;

this sets aside storage for it. The variable must also be declared in each func-tion

that wants to access it; this states the type of the variable. The declaration may

be an explicit extern statement or may be implicit from context. To make the

discussion concrete, let us rewrite the longest-line program with line, longest, and

max as external variables. This requires changing the calls, declarations, and bodies

of all three functions.

32 A TUTORIAL INTRODUCTION CHAPTER I

#include

<stdio.h>

#define

MAXLINE 1000

/* maximum

input

line

size */

int max;

maximum length seen so far */

char line[MAXLINE];

current

input

line

char longest[MAXLINE]; I* longest line saved here */

int getline(void);

void copy(void);

/* print longest input line; specialized version */ main()

{

int len;

extern int max;

extern char longest[];

max = 0;

while ((len= getline()) > 0)

if (len > max) {

max = len;

copy();

}

if (max > 0) /* there was a line */ printf ( "%s",

longest);

return 0;

}

/* getline: specialized version */ int

getline(void)

{

int c, i;

extern char line[];

for (i = 0; i < MAXLINE-1

&& (c=getchar()) I= EOF && c I= '\n'; ++i)

line[i] = c;

if ( c == '\n') {

line[i] = c;

++i;

}

line[i] = '\0';

return i;

}

SECTION 1.10 EXTERNAL VARIABLES AND SCOPE 33

/* copy: specialized version */

void copy(void)

{

int i;

extern char line[], longest[];

i = 0;

while ( (longest[i] = line[i]) I= '\0')

++i;

}

The external variables in main, getline, and copy are defined by the first lines

of the example above, which state their type and cause storage to be allocated for

them. Syntactically, external definitions are just like definitions of local

variables, but since they occur outside of functions, the variables are exter-nal.

Before a function can use an external variable, the name of the variable must be

made known to the function. One way to do this is to write an extern declaration

in the function; the declaration is the same as before except for the added keyword

extern.

In certain circumstances, the extern declaration can be omitted. If the

definition of an external variable occurs in the source file before its use in a par-

ticular function, then there is no need for an extern declaration in the func-tion.

The extern declarations in main, getline and copy are thus redun-dant. In fact,

common practice is to place definitions of all external variables at the beginning

of the source file, and then omit all extern declarations.

If the program is in several source files, and a variable is defined in file/ and

used in file2 and file3, then extern declarations are needed in file2 and file3 to

connect the occurrences of the variable. The usual practice is to collect extern

declarations of variables and functions in a separate file, historically called a header,

that is included by #include at the front of each source file. The suffix . h is

conventional for header names. The functions of the standard library, for example,

are declared in headers like <stdio. h>. This topic is discussed at length in Chapter 4,

and the library itself in Chapter 7 and Appen-dix B.

Since the specialized versions of getline and copy have no arguments, logic

would suggest that their prototypes at the beginning of the file should be getline (

} and copy( }. But for compatibility with older C programs the standard takes an

empty list as an old-style declaration, and turns off all argu-ment list checking; the

word void must be used for an explicitly empty list. We will discuss this further in

Chapter 4.

You should note that we are using the words definition and declaration care-

fully when we refer to external variables in this section. "Definition" refers to

the place where the variable is created or assigned storage; "declaration" refers

to places where the nature of the variable is stated but no storage is allocated.

By the way, there is a tendency to make everything in sight an extern vari-

able because it appears to simplify communications-argument lists are short

34 A TUTORIAL INTRODUCTION CHAPTER 1

and variables are always there when you want them. But external variables are

always there even when you don't want them. Relying too heavily on external

variables is fraught with peril since it leads to programs whose data connections

are not at all obvious-variables can be changed in unexpected and even inad-

vertent ways, and the program is hard to modify. The second version of the

longest-line program is inferior to the first, partly for these reasons, and partly

because it destroys the generality of two useful functions by wiring into them the

names of the variables they manipulate.

At this point we have covered what might be called the conventional core of

C. With this handful of building blocks, it's possible to write useful programs of

considerable size, and it would probably be a good idea if you paused long

enough to do so. These exercises suggest programs of somewhat greater com-

plexity than the ones earlier in this chapter.

Exercise 1-20. Write a program detab that replaces tabs in the input with the

proper number of blanks to space to the next tab stop. Assume a fixed set of tab

stops, say every n columns. Should n be a variable or a symbolic parame-ter? D

Exercise 1-21. Write a program entab that replaces strings of blanks by the

minimum number of tabs and blanks to achieve the same spacing. Use the same

tab stops as for detab. When either a tab or a single blank would suffice to reach

a tab stop, which should be given preference? D

Exercise 1-22. Write a program to "fold" long input lines into two or more

shorter lines after the last non-blank character that occurs before the n-th column

of input. Make sure your program does something intelligent with very long

lines, and if there are no blanks or tabs before the specified column. D

Exercise 1-23. Write a program to remove all comments from a C program.

Don't forget to handle quoted strings and character constants properly. C com-

ments do not nest. D

Exercise 1-24. Write a program to check a C program for rudimentary syntax

errors like unbalanced parentheses, brackets and braces. Don't forget about

quotes, both single and double, escape sequences, and comments. (This pro-

gram is hard if you do it in full generality.) D

cHAPTER 2: Types, Operators, and Expressions

Variables and constants are the basic data objects manipulated in a program.

Declarations list the variables to be used, and state what type they have and

perhaps what their initial values are. Operators specify what is to be done to

them. Expressions combine variables and constants to produce new values. The

type of an object determines the set of values it can have and what operations can

be performed on it. These building blocks are the topics of this chapter.

The ANSI standard has made many small changes and additions to basic types

and expressions. There are now signed and unsigned forms of all integer types,

and notations for unsigned constants and hexadecimal character constants.

Floating-point operations may be done in single precision; there is also a long

double type for extended precision. String constants may be con-catenated at

compile time. Enumerations have become part of the language, formalizing a

feature of long standing. Objects may be declared const, which prevents them

from being changed. The rules for automatic coercions among arithmetic types

have been augmented to handle the richer set of types.

2.1 Variable Names

Although we didn't say so in Chapter 1, there are some restrictions on the

names of variables and symbolic constants. Names are rnade up of letters and

digits; the first character must be a letter. The underscore "-" counts as a letter; it

is sometimes useful for improving the readability of long variable names. Don't

begin variable names with underscore, however, since library rou-tines often use

such names. Upper case and lower case letters are distinct, so x and X are two

different names. Traditional C practice is to use lower case for variable names,

and all upper case for symbolic constants.

At least the first 31 characters of an internal name are significant. For

function names and external variables, the number may be less than 31, because

external names may be used by assemblers and loaders over which the language

has no control. For external names, the standard guarantees uniqueness only for

6 characters and a single case. Keywords like if, else, int, float, etc.,

36 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

are reserved: you can't use them as variable names. They must be in lower case.

It's wise to choose variable names that are related to the purpose of the vari-

able, and that are unlikely to get mixed up typographically. We tend to use short·

names for local variables, especially loop indices, and longer names for external

variables.

2.2 Data Types and Sizes

There are only a few basic data types in C:

char

int

float

double

a single byte, capable of holding one character

in the local character set.

an integer, typically reflecting the natural size

of integers on the host machine.

single-precision floating point.

double-precision floating point.

In addition, there are a number of qualifiers that can be applied to these basic

types. short and long apply to integers:

short int sh;

long int counter;

The word int can be omitted in such declarations, and typically is.

The intent is that short and long should provide different lengths of integers

where practical; int will normally be the natural size for a particular machine.

short is often 16 bits, long 32 bits, and int either 16 or 32 bits. Each compiler is

free to choose appropriate sizes for its own hardware, subject only to the

restriction that shorts and ints are at least 16 bits, longs are at least 32 bits, and

short is no longer than int, which is no longer than long.

The qualifier signed or unsigned may be applied to char or any integer.

unsigned numbers are always positive or zero, and obey the laws of arithmetic

modulo 2", where n is the number of bits in the type. So, for instance, if chars are

8 bits, unsigned char variables have values between 0 and 255, while signed

chars have values between -128 and 127 (in a two's complement machine).

Whether plain chars are signed or unsigned is machine-dependent, but printable

characters are always positive.

The type long double specifies extended-precision floating point. As with

integers, the sizes of floating-point objects are implementation-defined; float,

double and long double could represent one, two or three distinct sizes.

The standard headers <limits. h> and <float. h> contain symbolic con-stants

for all of these sizes, along with other properties of the machine and com-piler.

These are discussed in Appendix B.

Exercise 2-1. Write a program to determine the ranges of char, short, int,

SECTION 2.3 CONSTANTS 37

and long variables, both signed and unsigned, by printing appropriate values from

standard headers and by direct computation. Harder if you com-pute them: determine

the ranges of the various floating-point types. 0

2.3 Constants

An integer constant like 1234 is an int. A long constant is written with a terminal 1

(ell) or L, as in 123456789L; an integer too big to fit into an int will also be taken as a

long. Unsigned constants are written with a terminal u or U, and the suffix ul or UL

indicates unsigned long.

Floating-point constants contain a decimal point ( 12 3 . 4) or an exponent ( 1e-2) or

both; their type is double, unless suffixed. The suffixes for F indi-cate a float constant; 1

or L indicate a long double.

The value of an integer can be specified in octal or hexadecimal instead of decimal.

A leading 0 (zero) on an integer constant means octal; a leading Ox or OX means

hexadecimal. For example, decimal 31 can be written as 0 3 7 in octal and Ox 1 f or OX

1F in hex. Octal and hexadecimal constants may also be followed by L to make them

long and U to make them unsigned: OXFUL is an unsigned long constant with value 15

decimal.

A character constant is an integer, written as one character within single quotes,

such as 'x'. The value of a character constant is the numeric value of the character in the

machine's character set. For example, in the ASCII charac-ter set the character constant

'0' has the value 48, which is unrelated to the numeric value 0. If we write '0' instead of a

numeric value like 48 that depends on character set, the program is independent of the

particular value and easier to read. Character constants participate in numeric operations

just as any other integers, although they are most often used in comparisons with other

characters.

Certain characters can be represented in character and string constants by escape

sequences like \n (newline): these sequences look like two characters, but represent only

one. In addition, an arbitrary byte-sized bit pattern can be specified by

'\ooo'

where ooo is one to three octal digits (0 ... 7) or by

'\x.hh'

where hh is one or more hexadecimal digits (0 ... 9, a ... f, A. .. F). So we might write

#define VTAB '\013' /* ASCII vertical tab */

#define BELL '\007' /* ASCII bell character */ or, in hexadecimal,

38TYPES, OPERATORS AND EXPRESSIONS

CHAPTER 2

#define VTAB

'\xb'

/* ASCII vertical tab */

#define BELL

'\x7'

I* ASCII bell character */

The complete set of escape sequences is

alert (bell) character

backslash

backspace

question mark

formfeed

single quote

newline

double quote

carriage return

\ooo

octal number

horizontal tab

\xhh

hexadecimal number

vertical tab

The character constant '\.0' represents the character with value zero, the null

character. '\.0' is often written instead of 0 to emphasize the character nature of some

expression, but the numeric value is just 0.

A constant expression is an expression that involves only constants. Such

expressions may be evaluated during compilation rather than run-time, and accordingly

may be used in any place that a constant can occur, as in

#define MAXLINE 1000

char line[MAXLINE+1];

#define LEAP 1 /* in leap years */

int days[31+28+LEAP+31+30+31+30+31+31+30+31+30+31];

A string constant, or string literal, is a sequence of zero or more characters

surrounded by double quotes, as in

"I am a string"

"" /* the empty string */

The quotes are not part of the string, but serve only to delimit it. The same escape

sequences used in character constc:nts apply in strings; \." represents the double-quote

character. String constants can be concatenated at compile time:

"hello," "world"

is equivalent to

"hello, world"

This is useful for splitting long strings across several source lines.

Technically, a string constant is an array of characters. The internal representation of

a string has a null character '\.0' at the end, so the physical storage required is one more

than the number of characters written between the quotes. This representation means

that there is no limit to how long a string can be, but programs must scan a string

completely to determine its length. The standard library function strlen( s) returns the

length of its character

SECTION 2.3 CONSTANTS 39

string argument s, excluding the terminal '\0 '. Here is our version:

/* strlen: return length of s */ int strlen(char

s[])

{

int i;

i = 0;

while (s[i] I= '\0')

++i;

return i;

}

strlen and other string functions are declared in the standard header <string. h>.

Be careful to distinguish between a character constant and a string that con-

tains a single character: 'x' is not the same as 11 x 11 • The former is an integer, used

to produce the numeric value of the letter x in the machine's character set. The

latter is an array of characters that contains one character (the letter x) and a '\0'.

There is one other kind of constant, the enumeration constant. An

enumeration is a list of constant integer values, as in

enum boolean { NO, YES };

The first name in an enwn has value 0, the next 1, and so on, unless explicit

values are specified. If not all values are specified, unspecified values continue

the progression from the last specified value, as in the second of these examples:

enum escapes {BELL= '\a', BACKSPACE= '\b', TAB= '\t', NEWLINE= '\n', VTAB

= '\v', RETURN= '\r' };

enum months { JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG,

SEP, OCT, NOV, DEC };

I* FEB is 2, MAR is 3, etc. */

Names in different enumerations must be distinct. Values need not be distinct in

the same enumeration.

Enumerations provide a convenient way to associate constant values with

names, an alternative to #define with the advantage that the values can be

generated for you. Although variables of enwn types may be declared, com-pilers

need not check that what you store in such a variable is a valid value for the

enumeration. Nevertheless, enumeration variables offer the chance of checking

and so are often better than #defines. In addition, a debugger may be able to print

values of enumeration variables in their symbolic form.

40 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

2.4 Declarations

All variables must be declared before use, although certain declarations can

be made implicitly by context. A declaration specifies a type, and contains a list

of one or more variables of that type, as in

int lower, upper, step;

char c, line[1000];

Variables can be distributed among declarations in any fashion; the lists above

could equally well be written as

int lower;

int upper;

int step;

char c;

char line[1000];

This latter form takes more space, but is convenient for adding a comment to

each declaration or for subsequent modifications.

A variable may also be initialized in its declaration. If the name is followed

by an equals sign and an expression, the expression serves as an initializer, as in

char

esc

= '\\';

int

i =

int

limit = MAXLINE+1;

float eps = 1.0e-5;

If the variable in question is not automatic, the initialization is done once only,

conceptually before the program starts executing, and the initializer must be a

constant expression. An explicitly initialized automatic variable is initial-ized each

time the function or block it is in is entered; the initializer may be any expression.

External and static variables are initialized to zero by default. Automatic variables

for which there is no explicit initializer have undefined (i.e., garbage) values.

The qualifier const can be applied to the declaration of any variable to specify

that its value will not be changed. For an array, the const qualifier says that the

elements will not be altered.

const double e = 2.71828182845905; const

char msq[] = "warning: ";

The const declaration can also be used with array arguments, to indicate that the

function does not change that array:

int strlen(const char[));

The result is implementation-defined if an attempt is made to change a const.

SECTION 2.6 RELATIONAL AND LOGICAL OPERATORS 41

2.5 Arithmetic Operators

The binary arithmetic operators are +, -, *• I, and the modulus operator %.

Integer division truncates any fractional part. The expression

x%y

produces the remainder when x is divided by y, and thus is zero when y divides x

exactly. For example, a year is a leap year if it is divisible by 4 but not by 100,

except that years divisible by 400 are leap years. Therefore

if ((year% 4 == 0 && year% 100 I= 0) :: year% 400 == 0)

printf ( 11 %d is a leap year\n 11 , year) ;

else

printf ( 11 %d is not a leap year\n 11 , year) ;

The %operator cannot be applied to float or double. The direction of trun-cation

for I and the sign of the result for % are machine-dependent for negative

operands, as is the action taken on overflow or underflow.

The binary + and - operators have the same precedence, which is lower than

the precedence of *• I, and %, which is in turn lower than unary + and -.

Arithmetic operators associate left to right.

Table 2-1 at the end of this chapter summarizes precedence and associativity

for all operators.

2.6 Relational and Logical Operators

The relational operators are

> >= < <=

They all have the same precedence. Just below them in precedence are the

equality operators:

== I=

Relational operators have lower precedence than arithmetic operators, so an

expression like i < 1 im-1 is taken as i < ( 1 im- 1 ) , as would be expected.

More interesting are the logical operators &.&. and I I. Expressions connected

by &.&. or I I are evaluated left to right, and evaluation stops as soon as the truth

or falsehood of the result is known. Most C programs rely on these properties.

For example, here is a loop from the input function getline that we wrote in

Chapter 1:

for (i=O; i<lim-1 && (c=getchar()) I= '\n' && c I= EOF; ++i) s(i] = c;

Before reading a new character it is necessary to check that there is room to store

it in the array s, so the test i < lim-1 must be made first. Moreover, if this test fails,

we must not go on and read another character.

41 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

Similarly, it would be unfortunate if c were tested against EOF before getchar

is called; therefore the call and assignment must occur before the character in c

is tested.

The precedence of &.&. is higher than that of I I, and both are lower than

relational and equality operators, so expressions like

i<lim-1 && (c = qetchar()) I= '\n' && c I= EOF

need no extra parentheses. But since the precedence of I = is higher than

assignment, parentheses are needed in

(c = qetchar()) I= '\n'

to achieve the desired result of assignment to c and then comparison with • \n •.

By definition, the numeric value of a relational or logical expression is 1 if

the relation is true, and 0 if the relation is false.

The unary negation operator I converts a non-zero operand into 0, and a zero

operand into 1. A common use of I is in constructions like

if (!valid)

rather than

if (valid == 0)

It's hard to generalize about which form is better. Constructions like I valid read

nicely ("if not valid"), but more complicated ones can be hard to under-stand.

Exercise 2-2. Write a loop equivalent to the for loop above without using &.&.

or II. D

2. 7 Type Conversions

When an operator has operands of different types, they are converted to a

common type according to a small number of rules. In general, the only

automatic conversions are those that convert a "narrower" operand into a "wider"

one without losing information, such as converting an integer to floating point in

an expression like f + i. Expressions that don't make sense, like using a float as a

subscript, are disallowed. Expressions that might lose infor-mation, like

assigning a longer integer type to a shorter, or a floating-point type to an integer,

may draw a warning, but they are not illegal.

A char is just a small integer, so chars may be freely used in arithmetic

expressions. This permits considerable flexibility in certain kinds of character

transformations. One is exemplified by this naive implementation of the func-

tion atoi, which converts a string of digits into its numeric equivalent.

SECTION 2.7

TYPE CONVERSIONS

int

atoi: convert s to integer atoi(char s[])

{

int i, n;

n = 0;

for (i

0; s[i]

'0'

s[i]

'9'; ++i)

n = 10

(s[i) -

'0');

return n;

}

As we discussed in Chapter 1, the expression

s[i] -

'0'

gives the numeric value of the character stored in s [ i ] , because the values of ' 0

', ' 1 ', etc., form a contiguous increasing sequence.

Another example of char to int conversion is the function lower, which maps a

single character to lower case for the ASCII character set. If the char-acter is not

an upper case letter, lower returns it unchanged.

/* lower: convert c to lower case; ASCII only */ int lower(int c)

{

if (c >= 'A' && c <= 'Z')

return c + 'a ' - 'A' ;

else

return c;

}

This works for ASCII because corresponding upper case and lower case letters

are a fixed distance apart as numeric values and each alphabet is contiguous-

there is nothing but letters between A and z. This latter observation is not true of

the EBCDIC character set, however, so this code would convert more than just

letters in EBCDIC.

The standard header <ctype. h>, described in Appendix B, defines a family of

functions that provide tests and conversions that are independent of character set.

For example, the function to lower ( c ) returns the lower case value of c if c is

upper case, so tolower is a portable replacement for the function lower shown

above. Similarly, the test

c >= '0' && c <= '9'

can be replaced by

isdigit(c)

We will use the <ctype. h> functions from now on.

There is one subtle point about the conversion of characters to integers. The

language does not specify whether variables of type char are signed or unsigned

quantities. When a char is converted to an int, can it ever produce a negative

integer? The answer varies from machine to machine, reflecting

44 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

differences in architecture. On some machines a char whose leftmost bit is 1 will

be converted to a negative integer ("sign extension"). On others, a char is

promoted to an int by adding zeros at the left end, and thus is always positive.

The definition of C guarantees that any character in the machine's standard

printing character set will never be negative, so these characters will always be

positive quantities in expressions. But arbitrary bit patterns stored in character

variables may appear to be negative on some machines, yet positive on others.

For portability, specify signed or unsigned if non-character data is to be stored in

char variables.

Relational expressions like i > j and logical expressions connected by &.&.

and I I are defined to have value 1 if true, and 0 if false. Thus the assignment

d = c

'0'

'9'

sets d to 1 if c is a digit, and 0 if not. However, functions like isdigi t may return

any non-zero value for true. In the test part of if, while, for, etc., "true" just

means "non-zero," so this makes no difference.

Implicit arithmetic conversions work .much as expected. In general, if an

operator like + or * that takes two operands (a binary operator) has operands of

different types, the "lower" type is promoted to the "higher" type before the

operation proceeds. The result is of the higher type. Section 6 of Appendix A

states the conversion rules precisely. If there are no unsigned operands, how-

ever, the following informal set of rules will suffice:

If either operand is long double, convert the other to long double.

Otherwise, if either operand is double, convert the other to double.

Otherwise, if either operand is float, convert the other to float.

Otherwise, convert char and short to int.

Then, if either operand is long, convert the other to long.

Notice that floats in an expression are not automatically converted to double;

this is a change from the original definition. In general, mathematical functions

like those in <math. h> will use double precision. The main reason for using

float is to save storage in large arrays, or, less often, to save time on machines

where double-precision arithmetic is particularly expensive.

Conversion rules are more complicated when unsigned operands are

involved. The problem is that comparisons between signed and unsigned values

are machine-dependent,· because they depend on the sizes of the various integer

types. For example, suppose that int is 16 bits and long is 32 bits.

Then

-1L

< 1U, because 1U, which is an int, is promoted to a signed long.

But

-1L

> 1UL, because -1L is promoted to unsigned long and thus appears to

be a large positive number.

Conversions take place across assignments; the value of the right side is con-

verted to the type of the left, which is the type of the result.

SECTION 2.7 TYPE CONVERSIONS 45

A character is converted to an integer, either by sign extension or not, as

described above.

Longer integers are converted to shorter ones or to chars by dropping the

excess high-order bits. Thus in

int i;

char c;

i = c;

c = i;

the value of c is unchanged. This is true whether or not sign extension is

involved. Reversing the order of assignments might lose information, however.

If x is float and i is int, then x = i and i = x both cause conversions; float to int

causes truncation of any fractional part. When double is con-verted to float, whether

the value is rounded or truncated is implementation-dependent.

Since an argument of a function call is an expression, type conversions also

take place when arguments are passed to functions. In the absence of a func-tion

prototype, char and short become int, and float becomes double. This is why we

have declared function arguments to be int and double even when the function is

called with char and float.

Finally, explicit type conversions can be forced ("coerced") in any expres-

sion, with a l!nary operator called a cast. In the construction

( type-name ) expression

the expression is converted to the named type by the conversion rules above. The

precise meaning of a cast is as if the expression were assigned to a variable of the

specified type •. which is then used in place of the whole construction. For

example, the library routine sqrt expects a double argument, and will pro-duce

nonsense if inadvertently handed something else. (sqrt is declared in <math. h>.)

So if n is an integer, we can use

sqrt((double) n)

to convert the value of n to double before passing it to sqrt. Note that the cast

produces the value of n in the proper type; n itself is not altered. The cast

operator has the same high precedence as other unary operators, as summarized

in the table at the end of this chapter.

If arguments are declared by a function prototype, as they normally should be,

the declaration causes automatic coercion of any arguments when the func-tion is

called. Thus, given a function prototype for sqrt:

double sqrt(double);

the call

root2 = sqrt(2);

coerces the integer 2 into the double value 2. 0 without any need for a cast.

46 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

The standard library includes a portable implementation of a pseudo-random

number generator and a function for initializing the seed; the former illustrates a

cast:

unsigned long int next = 1;

/*rand: return pseudo-random integer on 0 .. 32767 */ int rand(void)

{

next = next * 1103515245 + 1234~;

return (unsigned int)(next/65536) % 32768;

}

/* srand: set seed for rand() */ void

srand(unsigned int seed)

{

next = seed;

}

Exercise 2-3. Write the function htoi ( s), which converts a string of hexa-decimal

digits (including an optional Ox or ox) into its equivalent integer value. The

allowable digits are 0 through 9, a through f, and A through F. D

2.8 Increment and Decrement Operators

C provides two unusual operators for incrementing and decrementing vari-ables.

The increment operator ++ adds 1 to its operand, while the decrement operator --

subtracts 1. We have frequently used ++ to increment variables, as in

if ( c == '\n' )

++nl;

The unusual aspect is that ++ and -- may be used either as prefix operators

(before the variable, as in ++n), or postfix (after the variable: n++). In both cases, the

effect is to increment n. But the expression ++n increments n before its value is

used, while n++ increments n after its value has been used. This means that in a

context where the value is being used, not just the effect, ++n and n++ are different.

If n is 5, then

x = n++;

sets x to 5, but

x = ++n;

sets x to 6. In both cases, n becomes 6. The increment and decrement opera-tors can

only be applied to variables; an expression like ( i + j ) + + is illegal.

SECTION 2.8 INCREMENT AND DECREMENT OPERATORS 47

In a context where no value is wanted, just the incrementing effect, as in

if (c == '\n')

nl++;

prefix and postfix are the same. But there are situations where one or the other is

specifically called for. For instance, consider the function squeeze ( s, c), which

removes all occurrences of the character c from the string s.

/* squeeze: delete all c from s */ void squeeze(char

s[], int c)

{

int i, j;

for (i = j = 0; s[i] I= '\0'; i++)

if (s[i] I= c)

s[j++] = s[i];

s[j] = '\0';

}

Each time a non-e occurs, it is copied into the current

is j incremented to be ready for the next character. to

j position, and only then

This is exactly equivalent

if (s[i] I= c) {

s[j] = s[i];

j++;

}

Another example of a similar construction comes from the getline func-tion

that we wrote in Chapter 1, where we can replace

if (c == '\n') {

s[i] = c;

++i;

}

by the more compact

if (c == '\n')

s[i++] = c;

As a third example, consider the standard function strcat ( s, t), which

concatenates the string t to the end of the string s. strcat assumes that there is

enough space in s to hold the combination. As we have written it, strcat returns

no value; the standard library version returns a pointer to the resulting string.

48 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

I* strcat: concatenate t to end of s; s must be big enough */ void strcat(char s(], chart(])

{

int i, j;

i = j = 0;

while

(s(i] I=

'\0')

find end of s */

i++;

while

( (s[i++]

= t[j++])

'\0')

}

As each character is copied from t to s, the postfix ++ is applied to both i and

j to make sure that they are in position for the next pass through the loop.

Exercise 2-4. Write an alternate version of squeeze ( s 1, s2) that deletes each

character in s 1 that matches any character in the string s2. 0

Exercise 2-5. Write the function any ( s 1 , s2), which returns the first location in

the string s 1 where any character from the string s2 occurs, or -1 if s 1 contains

no characters from s2. (The standard library function strpbrk does the same job

but returns a pointer to the location.) o

2.9 Bitwise Operators

C provides six operators for bit manipulation; these may only be applied to

integral operands, that is, char, short, int, and long, whether signed or unsigned.

bitwise AND

bitwise inclusive OR

bitwise exclusive OR

<< left shift

>> right shift

one's complement (unary)

The bitwise AND operator & is often used to mask off some set of bits; for

example,

n = n &. 0177;

sets to zero all but the low-order 7 bits of n.

The bitwise OR operator I is used to turn bits on:

x = x l SET_ON; '

sets to one in x the bits that are set to one in SET_ON.

The bitwise exclusive OR operator " sets a one in each bit position where its

operands have different bits, and zero where they are the same.

SECTION 2.9

BITWISE OPERATORS

One must distinguish the bitwise operators &. and

from the logical opera-

tors

&.&.

and I I, which imply left-to-right evaluation

of a truth value.

For

example, if xis 1 and y is 2, then x &. y is zero while x &.&. y is one.

The shift operators < < and > > perform left and right shifts of their left

operand by the number of bit positions given by the right operand, which must

be positive. Thus x < < 2 shifts the value of x left by two positions, filling

vacated bits with zero; this is equivalent to multiplication by 4. Right shifting an

unsigned quantity always fills vacated bits with zero. Right shifting a signed

quantity will fill with sign bits ("arithmetic shift") on some machines and with

O-bits ("logical shift") on others.

The unary operator - yields the one's complement of an integer; that is, it

converts each 1-bit into a O-bit and vice versa. For example,

X = X & -077

sets the last six bits of x to zero. Note that x &. -077 is independent of word

length, and is thus preferable to, for example, x &. 0177700, which assumes that

x is a 16-bit quantity. The portable form involves no extra cost, since -077 is a

constant expression that can be evaluated at compile time.

As an illustration of some of the bit operators, consider the function getbi ts (

x, p, n) that returns the (right adjusted) n-bit field of x that begins at position p.

We assume that bit position 0 is at the right end and that nand

p are sensible positive values. For example, getbi ts ( x, 4, 3 ) returns the three

bits in bit positions 4, 3 and 2, right adjusted.

/* getbits: get n bits from position p */ unsigned

getbits(unsigned x, int p, int n)

{

return (x >> (p+1-n)) & -(-0 << n);

}

The expression x > > ( p+ 1-n ) moves the desired field to the right end of the

word. -o is all 1-bits; shifting it left n bit positions with -O<<n places zeros in

the rightmost n bits; complementing that with - makes a mask with ones in the

rightmost n bits.

Exercise 2-6. Write a function setbi ts ( x, p, n, y) that returns x with the n bits

that begin at position p set to the rightmost n bits of y, leaving the other bits

unchanged. 0

Exercise 2-7. Write a function invert(x,p,n) that returns x with then bits that

begin at position p inverted (i.e., 1 changed into 0 and vice versa), leaving the

others unchanged. o

Exercise 2-8. Write a function rightrot ( x, n) that returns the value of the integer

x rotated to the right by n bit positions. 0

SO TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

2. 10 Assignment Operators and Expressions

Expressions such as

i = i + 2

in which the variable on the left hand side is repeated immediately on the right,

can be written in the compressed form

i += 2

The operator += is called an assignment operator.

Most binary operators (operators like + that have a left and right operand)

have a corresponding assignment operator op =, where op is one of

If expr

and expr

are expressions, then

is equivalent to

expr

(expr

)

op (expr

)

except that expr

is computed only once. Notice the parentheses around expr

X *= y + 1

means

X= X* (y + 1)

rather than

As an example, the function bi tcount counts the number of 1-bits in its

integer argument.

/* bitcount: count 1 bits in x */ int

bitcount(unsigned x)

{

int b;

for

(b = 0;

if (X &

x I=

01)

>>= 1)

b++;

return b;

}

Declaring the argument x to be unsigned ensures that when it is right-shifted,

vacated bits will be filled with zeros, not sign bits, regardless of the machine the

program is run on.

Quite apart from conciseness, assignment operators have the advantage that

they correspond better to the way people think. We say "add 2 to i" or

SECTION 2.11 CONDITIONAL EXPRESSIONS 51

"increment i by 2," not "take i, add 2, then put the result back in i." Thus the

expression i += 2 is preferable to i = i+2. In addition, for a complicated expression

yyval[yypv[p3+p4] + yypv[p1+p2]] += 2

the assignment operator makes the code easier to understand, since the reader

doesn't have to check painstakingly that two long expressions are indeed the

same, or to wonder why they're not. And an assignment operator may even help

a compiler to produce efficient code.

We have already seen that the assignment statement has a value and can

occur in expressions; the most common example is

while ((c = getchar()) I= EOF)

The other assignment operators (+=, -=, etc.) can also occur in expressions,

although this is less frequent.

In all such expressions, the type of an assignment expression is the type of its

left operand, and the value is the value after the assignment.

Exercise 2-9. In a two's complement number system, x &.= ( x-1 ) deletes the

rightmost 1-bit in x. Explain why. Use this observation to write a faster ver-sion

of bi tcount. 0

2. 11 Conditional Expressions

The statements

if (a > b)

z = a;

else

z = b;

compute in z the maximum of a and b. The conditional expression, written with

the ternary operator "? : ", provides an alternate way to write this and similar

constructions. In the expression

expr

? expr

: expr

the expression expr

is evaluated first. If it is non-zero (true), then the expres-

sion expr

is evaluated, and that is the value of the conditional expression.

Otherwise expr

is evaluated, and that is the value. Only one of expr

and expr

is evaluated. Thus to set z to the maximum of a and b,

z = (a > b) ? a : b; /* z = max(a, b) */

It should be noted that the conditional expression is indeed an expression,

and it can be used wherever any other expression can be. If expr

and expr 3

Sl TYPES, OPERA TORS AND EXPRESSIONS CHAPTER 2

are of different types, the type of the result is determined by the conversion rules

discussed earlier in this chapter. For example, iff is a float and n is an int, then

the expression

(n > 0) ? f : n

is of type float regardless of whether n is positive.

Parentheses are not necessary around the first expression of a conditional

expression, since the precedence of ? : is very low, just above assignment. They

are advisable anyway, however, since they make the condition part of the

expression easier to see.

The conditional expression often leads to succinct code. For example, this

loop prints n elements of an array, 10 per line, with each column separated by

one blank, and with each line (including the last) terminated by a newline.

for (i = 0; i < n; i++)

printf( 11 %6d%c 11 , a[i], (i%10==9 II i==n-1)? '\n' : ' ');

A newline is printed after every tenth element, and after the n-th. All other

elements are followed by one blank. This might look tricky, but it's more com-

pact than the equivalent if-else. Another good example is

printf(

You have %d item%s.\n

, n, n==1 ?

1111

);

Exercise 2-10. Rewrite the function lower, which converts upper case letters to

lower case, with a conditional expression instead of if-else. D

2.12 Precedence and Order of Evaluation

Table 2-1 summarizes the rules for precedence and associativity of all opera-

tors, including those that we have not yet discussed. Operators on the same line

have the same precedence; rows are in order of decreasing precedence, so, for

example, *• I, and %all have the same precedence, which is higher than that of

binary + and -. The "operator" ( ) refers to function call. The operators -> and . are

used to access members of structures; they will be covered in Chapter 6, along

with sizeof (size of an object). Chapter 5 discusses * (indirection through a

pointer) and &. (address of an object), and Chapter 3 discusses the comma

operator.

Note that the precedence of the bitwise operators&., ",and I falls below ==

and I =. This implies that bit-testing expressions like

if ( ( x & MASK) == 0) ...

must be fully parenthesized to give proper results.

C, like most languages, does not specify the order in which the operands of an

operator are evaluated. (The exceptions are &.&., I I, ? :, and ', '.) For example, in

a statement like

SECTION 2.12

PRECEDENCE AND ORDER OF EVALUATION

TABLE 2-1. PRECEDENCE AND ASSOCIATIVITY OF OPERATORS

OPERATORS

ASSOCIATIVITY

(

)

[ ]

left to right

+ -

& (type)

sizeof

right to left

left to right

? :

right to left

&= " =

:= <<= >>=

right to left

left to right

Unary +, -,and *have higher precedence than the binary forms.

X = f() + g();

f may be evaluated before q or vice versa; thus if either f or q alters a variable

on which the other depends, x can depend on the order of evaluation. Inter-

mediate results can be stored in temporary variables to ensure a particular

sequence.

Similarly, the order in which function arguments are evaluated is not speci-

fied, so the statement

printf(''%d %d\n", ++n, power(2, n));

can produce different results with different compilers, depending on whether n is

incremented before power is called. The solution, of course, is to write

++n;

printf("%d %d\n", n, power(2, n));

Function calls, nested assignment statements, and increment and decrement

operators cause "side effects" -some variable is changed as a by-product of the

evaluation of an expression. In any expression involving side effects, there can

be subtle dependencies on the order in which variables taking part in the expres-

sion are updated. One unhappy situation is typified by the statement

a[i] = i++;

The question is whether the subscript is the old value of i or the new.

54 TYPES, OPERATORS AND EXPRESSIONS CHAPTER 2

Compilers can interpret this in different ways, and generate different answers depending

on their interpretation. The standard intentionally leaves most such matters unspecified.

When side effects (assignment to variables) take place within an expression is left to the

discretion of the compiler, since the best order depends strongly on machine

architecture. (The standard does specify that all side effects on arguments take effect

before a function is called, but that would not help in the call to print£ above.)

The moral is that writing code that depends on order of evaluation is a bad

programming practice in any language. Naturally, it is necessary to know what things to

avoid, but if you don't know how they are done on various machines, you won't be

tempted to take advantage of a particular implementation.

cHAPTER 3: Control Flow

The control-flow statements of a language specify the order in which compu-

tations are performed. We have already met the most common control-flow

constructions in earlier examples; here we will complete the set, and be more

precise about the ones discussed before.

3. 1 Statements and Blocks

An expression such as x = 0 or i++ or print£( ... ) becomes a statement when it

is followed by a semicolon, as in

X = 0;

i++;

print£( ... );

In C, the semicolon is a statement terminator, rather than a separator as it is in

languages like Pascal.

Braces { and } are used to group declarations and statements together into a

compound statement, or block, so that they are syntactically equivalent to a single

statement. The braces that surround the statements of a function are one obvious

example; braces around multiple statements after an if, else, while, or for are

another. (Variables can be declared inside any block; we will talk about this in

Chapter 4.) There is no semicolon after the right brace that ends

a block.

3. 2 If-Else

The if-else statement is used to express decisions. Formally, the syntax is

if (expression)

statement

else

statement

56 CONTROL FLOW

CHAPTER 3

where the else part is optional.

The expression is evaluated; if it is true (that

is, if.

expression

has a non-zero

value), statement

executed.

If it is false

(expression is zero) and if there is an else part,

statement

is executed

instead.

Since an if simply tests the numeric value of an expression, certain coding

shortcuts are possible. The most obvious is writing

if (expression)

instead of

if (expression I= 0)

Sometimes this is natural and clear; at other times it can be cryptic.

Because the else part of an if-else is optional, there is an ambiguity when an

else is omitted from a nested if sequence. This is resolved by asso-ciating the

else with the closest previous else-less if. For example, in

if (n > 0)

if (a > b)

z = a;

else

z = b;

the else goes with the inner if, as we have shown by indentation. If that isn't what

you want, braces must be used to force the proper association:

if (n > 0) {

if (a > b)

z = a;

}

else

z = b;

The ambiguity is especially pernicious in situations like this:

if (n >=

= 0; i

for

< n;

i++)

( s [ i 1 >

0 )

{

print£ ( " ... " ) ;

return

}

printf("error -- n is negative\n");

The indentation shows unequivocally what you want, but the compiler doesn't

get the message, and associates the else with the inner if. This kind of bug can be

hard to find; it's a good idea to use braces when there are nested ifs.

By the way, notice that there is a semicolon after z = a in

SECTION 3.3

if (a > b)

z = a;

else

z = b;

This is because grammatically, a statement follows the if,

statement like "z = a;" is always terminated by a semicolon.

ELSE-IF 57

and an expression

3.3 Else-If

The construction

if (expression)

statement

else if (expression)

statement

else if (expression)

statement

else if (expression)

statement

else

statement

occurs so often that it is worth a brief separate discussion. This sequence of if

statements is the most general way of writing a multi-way decision. The

expressions are evaluated in order; if any expression is true, the statement asso-

ciated with it is executed, and this terminates the whole chain. As always, the code

for each statement is either a single statement, or a group in braces.

The last else part handles the "none of the above" or default case where none

of the other conditions is satisfied. Sometimes there is no explicit action for the

default; in that case the trailing

else

statement

can be omitted, or it may be used for error checking to catch an "impossible"

condition.

To illustrate a three-way decision, here is a binary search function that

decides if a particular value x occurs in the sorted array v. The elements of v

must be in increasing order. The function returns the position (a number between

0 and n-1) if x occurs in v, and -1 if not.

Binary search first compares the input value x to the middle element of the

array v. If x is less than the middle value, searching focuses on the lower half of

the table, otherwise on the upper half. In either case, the next step is to com-pare

x to the middle element of the selected half. This process of dividing the range in

two continues until the value is found or the range is empty.

CONTROL FLOW

CHAPTER 3

/* binsearch: find x in v[O] <= v[1] <= ••• <= v[n-1] */ int binsearch(int x, int v[], int

{

int

low, high, mid;

low = 0;

high = n - 1;

while (low <= high) {

mid = (low+high) I 2;

if (x < v[mid])

high = mid - 1 ;

else if (x > v[mid])

low = mid + 1;

else /* found match */

return mid;

}

return -1;

/*no match*/

}

The fundamental decision is whether xis less than, greater than, or equal to the

middle element v[mid] at each step; this is a natural for else-if.

Exercise 3-1. Our binary search makes two tests inside the loop, when one would

suffice (at the price of more tests outside). Write a version with only one test

inside the loop and measure the difference in run-time. D

3.4 Switch

The switch statement is a multi-way decision that tests whether an expres-sion

matches one of a number of constant integer values, and branches accord-ingly.

switch (expression ) {

case const-expr: statements

default: statements

}

Each case is labeled by one or more integer-valued constants or constant expres-

sions. If a case matches the expression value, execution starts at that case. All

case expressions must be different. The case labeled default is executed if none of

the other cases are satisfied. A default is optional; if it isn't there and if none of

the cases match, no action at all takes place. Cases and the default clause can

occur in any order.

In Chapter 1 we wrote a program to count the occurrences of each digit,

white space, and all other characters, using a sequence of if ... else if ... else. Here

is the same program with a switch:

SECTION 3.4 SWITCH 59

#include

<stdio.h>

main()

/*count digits, white

space,

others*/

{

int c, i, nwhite, nother, ndigit[10];

nwhite = nother = 0;

for

(i = 0;

< 10; i++)

ndigit[i)

= 0;

while ((c

= getchar())

I= EOF) {

switch

(c)

{

case '0': case '1': case '2': case '3': case '4':

case

'5': case '6': case '7': case '8': case '9':

ndigit[c-'0']++;

break;

case

'\n':

case

'\t':

nwhite++;

break;

default:

nother++;

break;

}

=");

printf("digits

for

(i =

0; i < 10; i++)

printf(" "d", ndigit[i]);

printf(",

white space

%d,

other

= %d\n",

nwhi te , nother ) ;

return 0;

}

The break statement causes an immediate exit from the switch. Because cases

serve just as labels, after the code for one case is done, execution falls through to

the next unless you take explicit action to escape. break and return are the most

common ways to leave a switch. A break statement can also be used to force an

immediate exit from while, for, and do loops, as will be discussed later in this

chapter.

Falling through cases is a mixed blessing. On the positive side, it allows

several cases to be attached to a single action, as with the digits in this example.

But it also implies that normally each case must end with a break to prevent

falling through to the next. Falling through from one case to another is not

robust, being prone to disintegration when the program is modified. With the

exception of multiple labels for a single computation, fall-throughs should be

used sparingly, and commented.

As a matter of good form, put a break after the last case (the default here)

even though it's logically unnecessary. Some day when another case gets added

at the end, this bit of defensive programming will save you.

60 CONTROL FLOW CHAPTER 3

Exercise 3-2. Write a function escape ( s, t) that converts characters like newline

and tab into visible escape sequences like \n and \ t as it copies the string t to s.

Use a switch. Write a function for the other direction as well, converting escape

sequences into the real characters. D

3. 5 Loops-While and For

We have already encountered the while and for loops. In

while (expression )

statement

the expression is evaluated. If it is non-zero, statement is executed and expres-

sion is re-evaluated. This cycle continues until expression becomes zero, at

which point execution resumes after statement.

The for statement

for (expr

; expr

)

statement

is equivalent to

expr

;

while (expr

) {

statement

expr

;

}

except for the behavior of continue, which is described in Section 3.7.

Grammatically, the three components of a for loop are expressions. Most

commonly, expr

and expr

are assignments or function calls and expr

is a

relational expression. Any of the three parts can be omitted, although the semi-

colons must remain. If expr

or expr

is omitted, it is simply dropped from the

expansion. If the test, expr

, is not present, it is taken as permanently true, so

for ( ; ; ) {

}

is an "infinite" loop, presumably to be broken by other means, such as a break or

return.

Whether to use while or for is largely a matter of personal preference. For

example, in

while ((c = getchar()) == 1 1 I I c == 1 \n 1 I I c == 1 \t 1 ) I• skip white

space characters •I

there is no initialization or re-initialization, so the while is most natural.

The for is preferable when there is a simple initialization and increment, since

it keeps the loop control statements close together and visible at the top of

SECTION 3.5 LOOPS- WHILE AND FOR 61

the loop. This is most obvious in

for (i = 0; i < n; i++)

which is the c idiom for processing the first nelements of an array, the analog

of the Fortran DO loop or the Pascal for. The analogy is not perfect, however,

since the index and limit of a C for loop can be altered from within the loop, and

the index variable i retains its value when the loop terminates for any rea-son.

Because the components of the for are arbitrary expressions, for loops are not

restricted to arithmetic progressions. Nonetheless, it is bad style to force

unrelated computations into the initialization and increment of a for, which are

better reserved for loop control operations.

As a larger example, here is another version of atoi for converting a string to

its numeric equivalent. This one is slightly more general than the one in Chapter

2; it copes with optional leading white space and an optional + or - sign.

(Chapter 4 shows atof, which does the same conversion for floating-point

numbers.)

The structure of the program reflects the form of the input:

skip white space, if any

get sign, if any

get integer part and convert it

Each step does its part, and leaves things in a clean state for the next. The whole

process terminates on the first character that could not be part of a number.

#include <ctype.h>

/* atoi: convert s to integer; version 2 */ int atoi(char s[])

{

int i, n, sign;

for

= 0;

isspace(s[i]);

i++)

/* skip white space */

sign=

(s[i]

== ' - ') ? -1 : 1;

(s[i] ==

'+' I I s[i] ==

' - ')

/*skip sign*/

i++;

for

= 0;

isdigit(s[i]);

i++)

n + (s[i] - '0');

return

sign

* n;

}

The standard library provides a more elaborate function strtol for conversion of

strings to long integers; see Section 5 of Appendix B.

The advantages of keeping loop control centralized are even more obvious

when there are several nested loops. The following function is a Shell sort for

sorting an array of integers. The basic idea of this sorting algorithm, which was

62 CONTROL FLOW CHAPTER 3

invented in 1959 by D. L. Shell, is that in early stages, far•apart elements are

compared, rather than adjacent ones as in simpler interchange sorts. This tends to

eliminate large amounts of disorder quickly, so later stages have less work to do.

The interval between compared elements is gradually decreased to one, at which

point the sort effectively becomes an adjacent interchange method.

I• shellsort: sort v[O] .•. v[n-1] into increasing order •I void shellsort(int v[], int

{

int gap, i, j, temp;

for (gap = nl2; gap > 0; gap /= 2)

for (i = gap; i < n; i++)

for (j=i-gap; j>=O && v[j]>v[j+gap]; j-=gap) { temp= v[j];

v[j] = v[j+gap];

v[j+gap] = temp;

}

There are three nested loops. The outermost controls the gap between com-pared

elements, shrinking it from n/2 by a factor of two each pass until it becomes

zero. The. middle loop steps along the elements. The innermost loop compares

each pair of elements that is separated by gap and reverses any that are out of

order. Since gap is eventually reduced to one, all elements are even-tually

ordered correctly. Notice how the generality of the for makes the outer loop fit

the same form as the others, even though it is not an arithmetic progres-sion.

One final C operator is the comma " , ", which most often finds use in the for

statement. A pair of expressions separated by a comma is evaluated left to right, and

the type and value of the result are the type and value of the right operand. Thus in a

for statement, it is possible to place multiple expressions in the various parts, for

example to process two indices in parallel. This is illus-trated in the function reverse

( s ) , which reverses the string s in place.

#include <string.h>

I• reverse: reverse string s in place •I void reverse(char

s[])

{

int c, i, j;

for (i = 0, j = strlen(s)-1; i < j; i++, j--) {

c = s[i]; s[i] =

s[j]; s[j] = c;

}

SECTION 3.6 LOOPS-DO-WHILE 63

The commas that separate function arguments, variables in declarations, etc., are not

comma operators, and do not guarantee left to right evaluation.

Comma operators should be used sparingly. The most suitable uses are for

constructs strongly related to each other, as in the for loop in reverse, and in macros

where a multistep computation has to be a single expression. A comma expression

might also be appropriate for the exchange of elements in reverse, where the

exchange can be thought of as a single operation:

for (i

= 0, j = strlen(s)-1; i = s[i], s[i]

= s(j], s(j]

< j;

= c;

i++,

j--)

Exercise 3-3. Write a function expand ( s 1 , s2) that expands shorthand nota-tions

like a-z in the string s 1 into the equivalent complete list abc...xyz in s2. Allow for

letters of either case and digits, and be prepared to handle cases like a-b-c and a-z0-

9 and -a-z. Arrange that a leading or trailing - is taken literally. D

3. 6 Loops- Do-while

As we discussed in Chapter 1, the while and for loops test the termination

condition at the top. By contrast, the third loop in C, the do-while, tests at the

bottom after making each pass through the loop body; the body is always executed

at least once.

The syntax of the do is

statement

while (expression);

The statement is executed, then expression is evaluated. If it is true, statement is

evaluated again, and so on. When the expression becomes false, the loop ter-

minates. Except for the sense of the test, do-while is equivalent to the Pascal repeat-

until statement.

Experience shows that do-while is much less used than while and for.

Nonetheless, from time to time it is valuable, as in the following function i toa,

which converts a number to a character string (the inverse of atoi). The job is

slightly more complicated than might be thought at first, because the easy methods

of generating the digits generate them in the wrong order. We have chosen to

generate the string backwards, then reverse it.

64 CONTROL FLOW CHAPTER 3

I* itoa: convert n to characters in s */ void itoa(int n,

chars[])

{

int i, sign;

if ((sign= n)

n = -n;

< 0)

/*record sign*/

/* make n positive */

do { /* generate digits in reverse order */

s[i++] = n% 10 + •o•; I* get next digit*/

} while ((n I= 10) > 0); /*delete it*/

if (sign < 0)

s[i++] = •-•;

s[i] = '\0';

· reverse(s);

}

The do-while is necessary, or at least convenient, since at least one character must

be installed in the array s, even if n is zero. We also used braces around the single

statement that makes up the body of the do-while, even though they are

unnecessary, so the hasty reader will not mistake the while part for the beginning

of a while loop.

Exercise 3-4. In a two's complement number representation, our version of i toa

does not handle the largest negative number, that is, the value of n equal to -

(2wordsizc-l). Explain why not. Modify it to print that value correctly, regardless of

the machine on which it runs. 0

Exercise 3-5. Write the function i tob ( n, s, b) that converts the integer n into a

base b character representation in the string s. In particular, i tob ( n , s , 16 )

formats n as a hexadecimal integer in s. o

Exercise 3-6. Write a version of i toa that accepts three arguments instead of two.

The third argument is a minimum field width; the converted number must be

padded with blanks on the left if necessary to make it wide enough. 0

3. 7 Break and Continue

It is sometimes convenient to be able to exit from a loop other than by test-ing at

the top or bottom. The break statement provides an early exit from for, while, and do,

just as from switch. A break causes the innermost enclosing loop or switch to be

exited immediately.

The following function, trim, removes trailing blanks, tabs, and newlines

from the end of a string, using a break to exit from a loop when the rightmost

non-blank, non-tab, non-newline is found.

SECTION 3.8 GOTO AND LABELS 65

I* trim: remove trailing blanks, tabs, newlines */ int trim(char s[])

{

int n;

for (n = strlen(s)-1; n >= 0; n--)

if (s[n] I= ' ' &.&. s[n] I= '\t' &.&. s[n] I= '\n')

break;

s[n+1] = '\0';

return n;

}

strlen returns the length of the string. The for loop starts at the end and scans

backwards looking for the first character that is not a blank or tab or newline.

The loop is broken when one is found, or when n becomes negative (that is,

when the entire string has been scanned). You should verify that this is correct

behavior even when the string is empty or contains only white space characters.

The continue statement is related to break, but less often used; it causes the

next iteration of the enclosing for, while, or do loop to begin. In the while and

do, this means that the test part is executed immediately; in the for, control

passes to the increment step. The continue statement applies only to loops, not to

switch. A continue inside a switch inside a loop causes the next loop iteration.

As an example, this fragment processes only the non-negative elements in the

array a; negative values are skipped.

for (i

= 0; i < n; i++) {

(a[i] < 0) /* skip negative elements

continue;

/* do positive elements */

}

The continue statement is often used when the part of the loop that follows is

complicated, so that reversing a test and indenting another level would nest the

program too deeply.

3.8 Goto and Labels

C provides the infinitely-abusable goto statement, and labels to branch to.

Formally, the goto is never necessary, and in practice it is almost always easy to

write code without it. We have not used goto in this book.

Nevertheless, there are a few situations where gotos may find a place. The

most common is to abandon processing in some deeply nested structure, such as

breaking out of two or more loops at once. The break statement cannot be used

directly since it only exits from the innermost loop. Thus:

66 CONTROL FLOW CHAPTER 3

for ( ... )

for ( ... ) {

if (disaster)

goto error;

}

error:

clean up the mess

This organization is handy if the error-handling code is non-trivial, and if errors

can occur in several places.

A label has the same form as a variable name, and is followed by a colon. It

can be attached to any statement in the same function as the goto. The scope of a

label is the entire function.

As another example, consider the problem of determining whether two arrays

a and b have an element in common. One possibility is

for

= 0;

i <

i++)

for

= 0;

m; j++)

(a[i]

b[j])

goto

found;

I• didn't find any common element •I

found:

== b[j] •I

I•

got

one:

a[i]

Code involving a goto can always be written without one, though perhaps at

the price of some repeated tests or an extra variable. For example, the array

search becomes

found

= 0;

for

= 0;

&& !found;

i++)

for (j

m && !found;

j++)

(a[i]

b[j])

(found)

found

= 1;

== b[j-1]

I•

got

one:

a[i-1]

•I

else

I•

didn't

find

any

common

element •I

With a few exceptions like those cited here, code that relies on goto state-

ments is generally harder to understand and to maintain than code without gotos.

Although we are not dogmatic about the matter, it does seem that goto statements

should be used rarely, if at all.

cHAPTER 4: Functions and Program Structure

Functions break large computing tasks into smaller ones, and enable people

to build on what others have done instead of starting over from scratch.

Appropriate functions hide details of operation from parts of the program that

don't need to know about them, thus clarifying the whole, and easing the pain of

making changes.

C has been designed to make functions efficient and easy to use; C programs

generally consist of many small functions rather than a few big ones. A pro-gram

may reside in one or more source files. Source files may be compiled separately

and loaded together, along with previously compiled functions from libraries.

We will not go into that process here, however, since the details vary from

system to system.

Function declaration and definition is the area where the ANSI standard has

made the most visible changes to C. As we saw first in Chapter 1, it is now

possible to declare the types of arguments when a function is declared. The

syntax of function definition also changes, so that declarations and definitions

match. This makes it possible for a compiler to detect many more errors than it

could before. Furthermore, when arguments are properly declared, appropriate

type coercions are performed automatically.

The standard clarifies the rules on the scope of names; in particular, it

requires that there be only one definition of each external object. Initialization is

more general: automatic arrays and structures may now be initialized.

The C preprocessor has also been enhanced. New preprocessor facilities

include a more complete set of conditional compilation directives, a way to

create quoted strings from macro arguments, and better control over the macro

expansion process.

4. 1 Basics of Functions

To begin, let us design and write a program to print each line of its input that

contains a particular "pattern" or string of characters. (This is a special case of

the UNIX program grep.) for example, searching for the pattern of

68 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

letters "ould" in the set of lines

Ah Love! could you and I with Fate conspire To qrasp this

sorry Scheme of Thinqs entire, Would not we shatter it to

bits -- and then Re-mould it nearer to the Heart's Desire!

will produce the output

Ah Love! could you and I with Fate conspire

Would not we shatter it to bits -- and then

Re-mould it nearer to the Heart's Desire!

The job falls neatly into three pieces:

while ( there·s another line)

if (the line contains the pattern)

print it

Although it's certainly possible to put the code for all of this in main, a better

way is to use the structure to advantage by making each part a separate function.

Three small pieces are easier to deal with than one big one, because irrelevant

details can be buried in the functions, and the chance of unwanted interactions is

minimized. And the pieces may even be useful in other pro-grams.

"While there's another line" is getline, a function that we wrote in Chapter 1,

and "print it" is printf, which someone has already provided for us. This means

we need only write a routine to decide whether the line contains an occurrence of

the pattern.

We can solve that problem by writing a function strindex ( s, t) that returns

the position or index in the string s where the string t begins, or -1 if s doesn't

contain t. Because C arrays begin at position zero, indexes will be zero or positive,

and so a negative value like -1 is convenient for signaling failure. When we later

need more sophisticated pattern matching, we only have to replace strindex; the rest

of the code can remain the same. (The standard library provides a function strstr that

is similar to strindex, except that it returns a pointer instead of an index.>

Given this much design, filling in the details of the program is straightfor-

ward. Here is the whole thing, so you can see how the pieces fit together. For

now, the pattern to be searched for is a literal string, which is not the most gen-

eral of mechanisms. We will return shortly to a discussion of how to initialize

character arrays, and in ChapterS will show how to make the pattern a param-

eter that is set when the program is run. There is also a slightly different ver-sion

of getline; you might find it instructive to compare it to the one in Chapter 1.

SECTION 4.1 BASICS OF FUNCTIONS 69

#include <stdio.h>

#define MAXLINE 1000 /* maximum input

line length */

int getline(char line[], int max);

int strindex(char source[], char searchfor[]);

char pattern[] = "ould"; I* pattern to search for */

/* find all lines matching pattern */ main()

{

char line[MAXLINE];

int found = 0;

while (getline(line, MAXLINE) > 0)

if (strindex(line, pattern) >= 0) { print£ (

"%s", line) ;

found++;

}

return found;

}

/* getline: get line into s, return length */ int getline(char s(],

int lim)

{

int c, i;

i = 0;

while (--lim> 0 && (c=getchar()) I= EOF && c I= '\n') s[i++] = c;

if (c == '\n')

s[i++] = c;

s[i] = '\0';

return i;

}

/* strindex: return index of t in s, -1 if none */ int strindex(char s[],

chart[])

{

int i, j, k;

for (i = 0; s[i] I= '\0'; i++) {

for (j=i, k=O; t[k]l='\0' && s[j]==t[k]; j++, k++)

if (k > 0 && t[k] == '\0')

return i;

}

return -1;

}

Each function definition has the form

70 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

return-type function-name (argument declarations)

{

declarations and statements

}

Various parts may be absent; a minimal function is

dummy() {}

which does nothing and returns nothing. A do-nothing function like this is

sometimes useful as a place holder during program development. If the return

type is oq1itted, int is assumed.

A program is just a set of definitions of variables and functions. Communi-

cation between the functions is by arguments and values returned by the func-

tions, and through external variables. The functions can occur in any order in the

source file, and the source program can be split into multiple files, so long as no

function is split.

The return statement is the mechanism for returning a value from the called

function to its caller. Any expression can follow return:

return expression;

The expression will be converted to the return type of the function if necessary.

Parentheses are often used around the expression, but they are optional.

The calling function is free to ignore the returned value. Furthermore, there

need be no expression after return; in that case, no value is returned to the caller.

Control also returns to the caller with no value when execution "falls off the end"

of the function by reaching the closing right brace. It is not illegal, but probably a

sign of trouble, if a function returns a value from one place and no value from

another. In any case, if a function fails to return a value, its "value" is certain to

be garbage.

The pattern-searching program returns a status from main, the number of

matches found. This value is available for use by the environment that called the

program.

The mechanics of how to compile and load a C program that resides on mul-

tiple source files vary from one system to the next. On the UNIX system, for

example, the cc command mentioned in Chapter 1 does the job. Suppose that the

three functions are stored in three files called main. c, get line. c, and strindex. c.

Then the command

cc main.c getline.c strindex.c

compiles the three files, placing the resulting object code in files main. o, getline.

o, and strindex. o, then loads them all into an executable file called a. out. If there is

an error, say in main. c, that file can be recompiled by itself and the result loaded

with the previous object files, with the command

cc main.c getline.o strindex.o

The cc command uses the " . c" versus " . o" naming convention to distinguish

SECTION 4.2 FUNCTIONS RETURNING NON-INTEGERS 71

source files from object files.

Exercise 4-1. Write the function strrindex ( s, t), which returns the position of

the rightmost occurrence of t in s, or -1 if there is none. D

4.2 Functions Returning Non-integers

So far our examples of functions have returned either no value (void) or an

int. What if a function must return some other type? Many numerical func-tions

like sqrt, sin, and cos return double; other specialized functions return other

types. To illustrate how to deal with this, let us write and use the function a tof (

s ) , which converts the string s to its double-precision floating-point equivalent.

atof is an extension of atoi, which we showed versions of in Chapters 2 and 3. It

handles an optional sign and decimal point, and the pres-ence or absence of

either integer part or fractional part. Our version is not a high-quality input

conversion routine; that would take more space than we care to use. The

standard library includes an a tof; the header < stdlib. h> declares it.

First, atof itself must declare the type of value it returns, since it is not int.

The type name precedes the function name:

#include <ctype.h>

I* atof: convert string s to double */ double atof(char s[])

{

double val, power;

int i, sign;

for (i = 0; isspace(s[i]); i++) /*skip white space*/

sign = ( s [ i] == ' - ' ) ? -1 : 1 ;

if (s[i] == '+' I I s[i] == ' - ')

i++;

for (val= 0.0; isdigit(s[i]); i++) val= 10.0 *val +

(s[i] - '0');

if (s[i] == ' . ')

i++;

for (power= 1.0; isdigit(s[i]); i++) { val= 10.0 *val+

(s[i] - '0'); power *= 10.0;

}

return sign * val I power;

}

Second, and just as important, the calling routine must know that atof returns

a non-int value. One way to ensure this is to declare atof explicitly

72 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

in the calling routine. The declaration is shown in this primitive calculator

(barely adequate for check-book balancing), which reads one number per line,

optionally preceded by a sign, and adds them up, printing the running sum after

each input:

#include <stdio.h>

#define MAXLINE 100

/* rudimentary calculator */

main()

{

double sum, atof(char []);

char line[MAXLINE];

int getline(char line[], int max);

sum = 0;

while (getline(line, MAXLINE) > 0) printf("\t%g\n",

sum+= atof(line));

return 0;

}

The declaration

double sum, atof(char []);

says that sum is a double variable, and that atof is a function that takes one char [

] argument and returns a double.

The function atof must be declared and defined consistently. If atof itself and

the call to it in main have inconsistent types in the same source file, the error will

be detected by the compiler. But if (as is more likely) atof were compiled

separately, the mismatch would not be detected, atof would return a double that

main would treat as an int, and meaningless answers would result.

In the light of what we have said about how declarations must match defini-

tions, this might seem surprising. The reason a mismatch can happen is that if

there is no function prototype, a function is implicitly declared by its first

appearance in an expression, such as

sum += atof(line)

If a name that has not been previously declared occurs in an expression and is

followed by a left parenthesis, it is declared by context to be a function name, the

function is assumed to return an int, and nothing is assumed about its arguments.

Furthermore, if a function declaration does not include arguments, as in

double atof ( ) ;

that too is taken to mean that nothing is to be assumed about the arguments of a

to£; all parameter checking is turned off. This special meaning of the empty

SECTION 4.3 EXTERNAL VARIABLES 73

argument list is intended to permit older C programs to compile with new com-

pilers. But it's a bad idea to use it with new programs. If the function takes

arguments, declare them; if it takes no arguments, use void.

Given atof, properly declared, we could write atoi (convert a string to int) in

terms of it:

/* atoi: convert string s to integer using atof */ int atoi(char s[])

{

double atof(char s[]);

return (int) atof(s);

}

Notice the structure of the declarations and the return statement. The value of the

expression in

return expression ;

is converted to the type of the function before the return is taken. Therefore, the

value of atof, a double, is converted automatically to int when it appears in this

return, since the function atoi returns an int. This opera-tion does potentially

discard information, however, so some compilers warn of it. The cast states

explicitly that the operation is intended, and suppresses any warning.

Exercise 4-2. Extend atof to handle scientific notation of the form

123.45e-6

where a floating-point number may be followed by e or E and an optionally

signed exponent. D

4.3 External Variables

A C program consists of a set of external objects, which are either variables

or functions. The adjective "external" is used in contrast to "internal," which

describes the arguments and variables defined inside functions. External vari-

ables are defined outside of any function, and are thus potentially available to

many functions. Functions themselves are always external, because C does not

allow functions to be defined inside other functions. By default, external vari-

ables and functions have the property that all references to them by the same

name, even from functions compiled separately, are references to the same thing.

(The standard calls this property external linkage.) In this sense, exter-nal

variables are analogous to Fortran COMMON blocks or variables in the outermost

block in Pascal. We will see later how to define external variables and functions

that are visible only within a single source file.

74 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER4

Because external variables are globally accessible, they provide an

alternative to function arguments and return values for communicating data

between func-tions. Any function may access an external variable by referring to

it by name, if the name has been declared somehow.

If a large number of variables must be shared among functions, external

variables are more convenient and efficient than long argument lists. As pointed out

in Chapter 1, however, this reasoning should be applied with some caution, for it can

have a bad effect on program structure, and lead to programs with too many data

connections between functions.

External variables are also useful because of their greater scope and lifetime.

Automatic variables are internal to a function; they come into existence when the

function is entered, and disappear when it is left. External variables, on the other

hand, are permanent, so they retain values from one function invocation to the

next. Thus if two functions must share some data, yet neither calls the other, it is

often most convenient if the shared data is kept in external variables rather than

passed in and out via arguments.

Let us examine this issue further with a larger example. The problem is to

write a calculator program that provides the operators +, -, *• and /. Because it is

easier to implement, the calculator will use reverse Polish notation instead of

infix. (Reverse Polish is used by some pocket calculators, and in languages like

Forth and Postscript.)

In reverse Polish notation, each operator follows its operands; an infix

expression like

(1 - 2) * (4 + 5)

is entered as

1 2 - 4 5 + *

Parentheses are not needed; the notation is unambiguous as long as we know

how many operands each operator expects.

The implementation is simple. Each operand is pushed onto a stack; when an

operator arrives, the proper number of operands (two for binary operators) is

popped, the operator is applied to them, and the result is pushed back onto the

stack. In the example above, for instance, 1 and 2 are pushed, then replaced by

their difference, -1. Next, 4 and 5 are pushed and then replaced by their sum,

9. The product of -1 and 9, which is -9, replaces them on the stack. The value

on the top of the stack is popped and printed when the end of the input line is

encountered.

The structure of the program is thus a loop that performs the proper opera-

tion on each operator and operand as it appears:

SECTION 4.3 EXTERNAL VARIABLES 75

while (next operator or operand is not end-of-file indicator)

if (number)

push it

else if (operator)

pop operands

do operation

push result

else if (newline)

pop and print top of stack

else

error

The operations of pushing and popping a stack are trivial, but by the time

error detection and recovery are added, they are long enough that it is better to

put each in a separate function than to repeat the code throughout the whole

program. And there should be a separate function for fetching the next input

operator or operand.

The main design decision that has not yet been discussed is where the stack

is, that is, which routines access it directly. One possibility is to keep it in main,

and pass the stack and the current stack position to the routines that push and pop it.

But main doesn't need to know about the variables that con-trol the stack; it only

does push and pop operations. So we have decided to store the stack and its

associated information in external variables accessible to the push and pop functions

but not to main.

Translating this outline into code is easy enough. If for now we think of the

program as existing in one source file, it will look like this:

#includes

, #defines

function declarations for main

main() { ... }

external variables for push and pop

void push(double f) { ... }

double pop(void) { ... }

int getop ( char s [ ] ) { .. . }

routines called by getop

Later we will discuss how this might be split into two or more source files.

The function main is a loop containing a big switch on the type of opera-tor or

operand; this is a more typical use of switch than the one shown in Sec-tion 3.4.

76FUNCTIONS AND PROGRAM STRUCTURE

CHAPTER 4

#include

<stdio.h>

#include

<stdlib.h>

for

atof()

#define MAXOP

100

max

size of

operand or operator

#define

NUMBER

•o•

signal that

a number was found

int getop(char [));

void push(double);

double pop(void);

I* reverse Polish calculator *I

main()

{

int type;

double op2 ; ·

char s [ MAXOP 1 ;

while ((type= getop(s)) I= EOF) { switch

(type) {

case NUMBER:

push(atof(s));

break;

case '+':

push ( pop ( ) + pop ( ) ) ;

break;

case • *':

push(pop() *pop());

break;

case • -•:

op2 = pop();

push(pop() - op2);

break;

case • I':

op2 = pop();

if (op2 I= 0.0)

push(pop() I op2);

else

printf("error: zero divisor\n"); break;

case '\n •:

print£( "\t%.8g\n"

pop());

break;

default:

printf("error: unknown command %s\n"

s); break;

}

return 0;

}

SECTION 4.3 EXTERNAL VARIABLES 77

Because + and * are commutative operators, the order in which the popped

operands are combined is irrelevant, but for - and I the left and right operands

must be distinguished. In

push(pop() - pop());

the order in which the two calls of pop are evaluated is not defined. To guarantee

the right order, it is necessary to pop the first value into a temporary variable as

we did in main.

#define MAXVAL 100

/* maximum depth of val stack */

int sp

= 0;

next free stack position */

double

val[MAXVAL];

value stack */

I* push: push f onto value stack */ void

push(double f)

{

if ( sp < MAXVAL)

val[sp++] = f;

else

printf("error: stack full. can't push %9''n". f);

}

/* pop: pop and return top value from stack */ double pop(void)

{

if (sp > 0)

return val[--sp];

else {

printf("error: stack empty'n");

return 0.0;

}

A variable is external if it is defined outside of any function. Thus the stack

and stack index that must be shared by push and pop are defined outside of these

functions. But main itself does not refer to the stack or stack position-the

representation can be hidden.

Let us now turn to the implementation of getop, the function that fetches the

next operator or operand. The task is easy. Skip blanks and tabs. If the next

character is not a digit or a decimal point, return it. Otherwise, collect a string of

digits (which might include a decimal point), and return NUMBER, the signal that

a number has been collected.

78 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

#include <ctype.h>

int getch(vo:j.d);

void ungetch(int);

/* getop: get next operator or numeric operand */ int getop(char s[])

{

int

while

((s[O] = c

= getch())

c == '\t')

s[1]

'\0';

if ( lisdigit(c) && c I=' . ')

return c;

/* not a

number

i = 0;

(isdigit(c))

/*collect

integer part*/

while (isdigit(s[++i] = c

= getch()))

(c == '.')

/*collect fraction part*/

while (isdigit(s[++i] = c

= getch()))

s [ i] =

'\0,;

I= EOF)

ungetch ( c ) ;

return

NUMBER;

}

What are getch and ungetch? It is often the case that a program cannot

determine that it has read enough input until it has read too much. One instance

is collecting the characters that make up a number: until the first non-digit is

seen, the number is not complete. But then the program has read one character

too far, a character that it is not prepared for.

The problem would be solved if it were possible to "un-read" the unwanted

character. Then, every time the program reads one character too many, it could

push it back on the input, so the rest of the code could behave as if it had never

been read. Fortunately, it's easy to simulate un-getting a character, by writing a

pair of cooperating functions. getch delivers the next input character to be

considered; ungetch remembers the characters put back on the input, so that

subsequent calls to getch will return them before reading new input.

How they work together is simple. ungetch puts the pushed-back charac-ters

into a shared buffer-a character array. getch reads from the buffer if there is

anything there, and calls getc~ar if the buffer is empty. There must also be an

index variable that records the J)osition of the current character in the buffer.

Since the buffer and the index are shared by getch and ungetch and must retain

their values between c~lls, they must be external to both routines. Thus we can

write getch, ungetch, and their shar~d variables as:

SECTION 4.3 EXTERNAL VARIABLES 79

#define BUFSIZE 100

char buf[BUFSIZE]; int

bufp = 0;

buffer for ungetch next

free position

buf

int getch(void)

get a (possibly pushed back)

character

{

return (bufp

? buf[--bufp]

: getchar();

}

void ungetch(int c)

push character back on

input

if (bufp >= BUFSIZE)

printf("ungetch: too many characters\n");

else

buf[bufp++] = c;

}

The standard library includes a function ungetc that provides one character of

push back; we will discuss it in Chapter 7. We have used an array for the push-

back, rather than a single character, to illustrate a more general approach.

Exercise 4-3. Given the basic framework, it's straightforward to extend the cal-

culator. Add the modulus (%) operator and provisions for negative numbers. 0

Exercise 4-4. Add commands to print the top element of the stack without pop-

ping, to duplicate it, and to swap the top two elements. Add a command to clear

the stack. o

Exercise 4-5. Add access to library functions like sin, exp, and pow. See <math.h>

in Appendix B, Section 4. 0

Exercise 4-6. Add commands for handling variables. (It's easy to provide twenty-

six variables with single-letter names.) Add a variable for the most recently

printed value. o

Exercise 4-7. Write a routine ungets ( s) that will push back an entire string onto

the input. Should ungets know about buf and bufp, or should it just use ungetch? 0

Exercise 4-8. Suppose that there will never be more than one character of

pushback. Modify getch and ungetch accordingly. 0

Exercise 4-9. Our getch and ungetch do not handle a pushed-back EOF correctly.

Decide what their properties ought to be if an EOF is pushed back, then

implement your design. o

Exercise 4-10. An alternate organization uses getline to read an entire input line;

this makes getch and ungetch unnecessary. Revise the calculator to use this

approach. 0

80 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

4.4 Scope Rules

The functions and external variables that make up a C program need not all

be compiled at the same time; the source text of the program may be kept in

several files, and previously compiled routines may be loaded from libraries.

Among the questions of interest are

• How are declarations written so that variables are properly declared during

compilation?

• How are declarations arranged so •that all the pieces will be properly con-

nected when the program is loaded'!

• How are declarations organized so there is only one copy?

• How are external variables initialized?

Let us discuss these topics by reorganizing the calculator program into several

files. As a practical matter, the calculator is too small to be worth splitting, but it

is a fine illustration of the issues that arise in larger programs.

The scope of a name is the part of the program within which the name can be

used. For an automatic variable declared at the beginning of a function, the scope

is the function in which the name is declared. Local variables of the same name

in different functions are unrelated. The same is true of the parameters of the

function, which are in effect local variables.

The scope of an external variable or a function lasts from the point at which it

is declared to the end of the file being compiled. For example, if main, sp, val,

push, and pop are defined in one file, in the order shown above, that is,

main() { ... }

int sp = 0;

double val[MAXVAL];

void push (double f ) { ... }

double pop(void) { ... }

then the variables sp and val may be used in push and pop simply by nam-ing

them; no further declarations are needed. But these names are not visible in main,

nor are push and pop themselves.

On the other hand, if an external variable is to be referred to before it is

defined, or if it is defined in a different source file from the one where it is being

used, then an extern declaration is mandatory.

It is important to distinguish between the declaration of an external variable and

its definition. A declaration announces the properties of a variable (pri-marily its

type); a definition also causes storage to be set aside. If the lines

int sp;

double val[MAXVAL];

appear outside of any function, they define the external variables sp and val,

SECTION 4.5 HEADER FILES 81

cause storage to be set aside, and also serve as the declaration for the rest of that

source file. On the other hand, the lines

extern int sp;

extern double val[];

declare for the rest of the source file that sp is an int and that val is a double array

(whose size is determined elsewhere), but they do not create the variables or reserve

storage for them.

There must be only one definition of an external variable among all the files

that make up the source program; other files may contain extern declarations to

access it. (There may also be extern declarations in the file containing the

definition.) Array sizes must be specified with the definition, but are optional

with an extern declaration.

Initialization of an external variable goes only with the definition.

Although it is not a likely organization for this program, the functions push

and pop could be defined in one file, and the variables val and sp defined and

initialized in another. Then these definitions and declarations would be neces-

sary to tie them together:

Infilel:

extern int sp;

extern double val[];

void push(double f) {

}

double pop(void) { ... }

Infile2:

int sp = 0;

double val[MAXVAL];

Because the extern declarations in filel lie ahead of and outsid~ the function

definitions, they apply to all functions; one set of declarations suffices for all of

filel. This same organization would also be needed if the definitions of sp and val

followed their use in one file.

4.5 Header Files

Let us now consider dividing the calculator program into several source files,

as it might be if each of the components were substantially bigger. The main

function would go in one file, which we will call main. c; push, pop, and their

variables go into a second file, stack. c; getop goes into a third, getop. c. Finally,

getch and ungetch go into a fourth file, getch. c; we separate them from the others

because they would come from a separately-compiled library in a realistic

program.

81 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

There is one more thing to worry about-the definitions and declarations

shared among the files. As much as possible, we want to centralize this, so that

there is only one copy to get right and keep right as the program evolves.

Accordingly, we will place this common material in a header file, calc. h, which

will be included as necessary. (The #include line is described in Sec-tion 4.11.)

The resulting program then looks like this:

main.c:

#include <stdio.h> #include

<stdlib.h> #include "calc.h"

#define MAXOP 100 main(

l {

}

calc.h:

#define NUMBER '0' void

push(doublel; double

pop(void); int getop(char

[]); int getch(void); void

ungetch(int);

getop.c:

#include <stdio.h>

#include <ctype.h>

#include "calc.h" getop()

getch.c:

#include <stdio.h> #define

BUFSIZE 100 char

buf[BUFSIZE]; int bufp =

int getchtvoid)

stack.c:

#include <stdio.h>

#include "calc.h"

#define MAXVAL

100

int sp = 0;

double val[MAXVAL];

void push(double) {

double pop(void)

void ungetch(int)

There is a tradeoff between the desire that each file have access only to the

information it needs for its job and the practical reality that it is harder to

maintain more header files. Up to some moderate program size, it is probably

best to have one header file that contains everything that is to be shared between

any two parts of the program; that is the decision we made here. For a much

larger program, more organization and more headers would be needed.

SECTION 4.7 REGISTER VARIABLES 83

4.6 Static Variables

The variables sp and val in stack. c, and buf and bufp in getch. c, are for the

private use of the functions in their respective source files, and are not meant to

be accessed by anything else. The static declaration, applied to an external

variable or function, limits the scope of that object to the rest of the source file

being compiled. External static thus provides a way to hide names like buf and

bufp in the getch-ungetch combination, which must be external so they can be

shared, yet which should not be visible to users of

getch and ungetch.

Static storage is specified by prefixing the normal declaration with the word

static. If the two routines and the two variables are compiled in one file, as in

static char buf[BUFSIZE]; static

int bufp = 0;

buffer for ungetch next

free position

in buf

int getch (void) { ... }

void ungetch( int c) { ... }

then no other routine will be able to access buf and bufp, and those names will

not conflict with the same names in other files of the same program. In the same

way, the variables that push and pop use for stack manipulation can be hidden, by

declaring sp and val to be static.

The external static declaration is most often used for variables, but it can be

applied to functions as well. Normally, function names are global, visible to any

part of the entire program. If a function is declared static, however, its name is

invisible outside of the file in which it is declared.

The static declaration can also be applied to internal variables. Internal static

variables are local to a particular function just as automatic variables are, but unlike

automatics, they remain in existence rather than coming and going each time the

function is activated. This means that internal static variables provide private,

permanent storage within a single function.

Exercise 4-11. Modify getop so that it doesn't need to use ungetch. Hint:

use an internal static variable. D

4.7 Register Variables

A register declaration advises the compiler that the variable in question will be

heavily used. The idea is that register variables are to be placed in machine registers,

which may result in smaller and faster programs. But com-pilers are free to ignore

the advice.

The register declaration looks like

84 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

and so on. The register declaration can only be applied to automatic vari-ables

and to the formal parameters of a function. In this latter case, it looks like

£(register unsigned m, register long n)

{

}

In practice, there are restrictions on register variables, reflecting the realities

of underlying hardware. Only a few variables in each function may be kept in

registers, and only certain types are allowed. Excess register declarations are

harmless, however, since the word register is ignored for excess or disallowed

declarations. And it is not possible to take the address of a register variable (a

topic to be covered in Chapter 5), regardless of whether the variable is actually

placed in a register. The specific restrictions on number and types of register

variables vary from machine to machine.

4.8 Block Structure

C is not a block-structured language in the sense of Pascal or similar

languages, because functions may not be defined within other functions. On the

other hand, variables can be defined in a block-structured fashion within a func-

tion. Declarations of variables (including initializations) may follow the left

brace that introduces any compound statement, not just the one that begins a

function. Variables declared in this way hide any identically named variables in

outer blocks, and remain in existence until the matching right brace. For exam-

ple, in

if (n >

{

int

declare

a new i *I

for

= 0;

i < n;

i++)

}

the scope of the variable i is the "true" branch of the if; this i is unrelated to any i

outside the block. An automatic variable declared and initialized in a block is

initialized each time the block is entered. A static variable is initial-ized only the

first time the block is entered.

Automatic variables, including formal parameters, also hide external vari-

ables and functions of the same name. Given the declarations

SECTION 4.9 INITIALIZATION 85

int x;

int y;

f(double x)

{

double y;

}

then within the function f, occurrences of x refer to the parameter, which is a

double; outside of f, they refer to the external int. The same is true of the variable y.

As a matter of style, it's best to avoid variable names that conceal names in an

outer scope; the potential for confusion and error is too great.

4.9 Initialization

Initialization has been mentioned in passing many times so far, but always

peripherally to some other topic. This section summarizes some of the rules, now

that we have discussed the various storage classes.

In the absence of explicit initialization, external and static variables are

guaranteed to be initialized to zero; automatic and register variables have unde-

fined (i.e., garbage) initial values.

Scalar variables may be initialized when they are defined, by following the

name with an equals sign and an expression:

int x = 1;

char squote = '\'';

long day = 1000L * 60L * 60L * 24L; /* milliseconds/day */

For external and static variables, the initializer must be a constant expression; the

initialization is done once, conceptually before the program begins execution.

For automatic and register variables, it is done each time the function or block is

entered.

For automatic and register variables, the initializer is not restricted to being a

constant: it may be any expression involving previously defined values, even

function calls. For example, the initializations of the binary search program in

Section 3.3 could be written as

int binsearch(int x, int v[], int n)

{

int low = 0;

int high = n - 1;

int mid;

}

instead of

FUNCTIONS AND PROGRAM STRUCTURE

CHAPTER 4

int

low, high, mid;

low

high

= 0;

= n - 1;

In effect, initializations of automatic variables are just shorthand for assignment

statements. Which form to prefer is largely a matter of taste. We have gen-erally

used explicit assignments, because initializers in declarations are harder to see and

further away from the point of use.

An array may be initialized by following its declaration with a list of initial-izers

enclosed in braces and separated by commas. For example, to initialize an array days

with the number of days in each month:

int days[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

When the size of the array is omitted, the compiler will compute the length by

counting the initializers, of which there are 12 in this case.

If there are fewer initializers for an array than the number specified, the missing

elements will be zero for extemal, static, and automatic variables. It is an error to

have too many initializers. There is no way to specify repetition of

an initializer, nor to initialize an element in the middle of an array without sup-

plying all the preceding values as well. · Character arrays are a special case of

initialization; a string may be used instead of the braces and commas notation:

char pattern []

"ould";

is a shorthand for the longer but equivalent

char pattern[] = { 'o', 'u', '1', 'd', '\0' };

In this case, the array size is five (four characters plus the terminating ' \0').

4.1 0 Recursion

C functions may be used recursively; that is, a function may call itself either

directly or indirectly. Consider printing a number as a character string. As we

mentioned before, the digits are generated in the wrong order: low-order digits are

available before high-order digits, but they have to be printed the other way around.

There are two solutions to this problem. One is to store the digits in an array as

they are generated, then print them in the reverse order, as we did with i toa in

Section 3.6. The alternative is a recursive solution, in which printd first calls itself to

cope with any leading digits, then prints the trailing digit. Again, this version can fail

on the largest negative number.

SECfiON 4.10 RECURSION 87

#include <stdio.h>

I* printd: print n in decimal *I void printd(int

{

if (n < 0) {

putchar('-');

n = -n;

if (n I 10)

printd(n I 10);

putchar(n% 10 + '0');

When a function calls itself recursively, each invocation gets a fresh set of all the

automatic variables, independent of the previous set. Thus in printd(123) the

firstprintdreceives the argumentn = 123. Itpasses 12 to a second printd, which in tum

passes 1 to a third. The third-level printd prints 1, then returns to the second level.

That printd prints 2, then returns to the first level. That one prints 3 and terminates.

Another good example of recursion is quicksort, a sorting algorithm developed

by C. A. R. Hoare in 1962. Given an array, one element is chosen and the others are

partitioned into two subsets-those less than the partition ele-ment and those greater

than or equal to it. The same process is then applied recursively to the two subsets.

When a subset has fewer than two elements, it doesn't need any sorting; this stops

the recursion.

Our version of quicksort is not the fastest possible, but it's one of the sim-plest.

We use the middle element of each subarray for partitioning.

I* qsort: sort v[left] ... v[right] into increasing order *I void qsort(int v[], int left, int

right)

{

int

last;

void swap(int v[], inti, int j);

(left >= right)

I* do nothing if array contains *I

return;

I* fewer than two elements *I

swap(v,

left, (left

right)l2); I*

move partition elem *I

last =

left;

v[O] *I

for

= left+1; i

right; i++)

partition *I

if (v(i] < v[left])

swap(v, ++last, i);

swap(v, left, last); I* restore partition elem *I

qsort(v, left, last-1);

qsort(v, last+l, right);

We moved the swapping operation into a separate function swap because it occurs

three times in qsort.

88 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER4

/* swap: interchange v[i] and v[j] */ void swap(int v[], int

i, int j)

{

int temp;

temp= v[i];

v[i] = v(j];

v[j] = temp;

}

The standard library includes a version of qsort that can sort objects of any type.

Recursion may provide no saving in storage, since somewhere a stack of the

values being processed must be maintained. Nor will it be faster. But recursive

code is more compact, and often much easier to write and understand than the

non-recursive equivalent. Recursion is especially convenient for recursively

defined data structures like trees; we will see a nice example in Section 6.5.

Exercise 4-12. Adapt the ideas of printd to write a recursive version of itoa; that

is, convert an integer into a string by calling a recursive routine. D

Exercise 4-13. Write a recursive version of the function reverse ( s), which

reverses the string s in place. D

4. 11 The C Preprocessor

C provides certain language facilities by means of a preprocessor, which is

conceptually a separate first step in compilation. The two most frequently used

features are #include, to include the contents of a file during compilation, and

#define, to replace a token by an arbitrary sequence of characters. Other features

described in this section include conditional compilation and macros with

arguments.

4. 11. 1 File Inclusion

File inclusion makes it easy to handle collections of #defines and declara-

tions (among other things). Any source line of the form

#include "filename"

#include <.filename>

is replaced by the contents of the file filename. If the filename is quoted, searching

for the file typically begins where the source program was found; if it is not

found there, or if the name is enclosed in < and >, searching follows an

implementation-defined rule to find the file. An included file may itself contain

SECTION 4.11 THE C PREPROCESSOR 89

#include lines.

There are often several #include lines at the beginning of a source file, to

include common #define statements and extern declarations, or to access the

function prototype declarations for library functions from headers like

< stdio. h>. (Strictly speaking, these need not be files; the details of how headers are

accessed are implementation-dependent.)

#include is the preferred W3.Y to tie the declarations together for a large

program. It guarantees that all the source files will be supplied with the same

definitions and variable declarations, and thus eliminates a particularly nasty kind of

bug. Naturally, when an included file is changed, all files that depend on it must be

recompiled.

4. 11.2 Macro Substitution

A definition has the form

#define name replacement text

It calls for a macro substitution of the simplest kind-subsequent occurrences of the

token name will be replaced by the replacement text. The name in a #define has the

same form as a variable name; the replacement text is arbi-trary. Normally the

replacement text is the rest of the line, but a long defini-tion may be continued onto

several lines by placing a \ at the end of each line to be continued. The scope of a

name defined with #define is from its point of definition to the end of the source file

being compiled. A definition may use previous definitions. Substitutions are made

only for tokens, and do not take place within quoted strings. For example, if YES is a

defined name, there would be no substitution in print£ ( "YES" ) or in YESMAN.

Any name may be defined with any replacement text. For example,

#define forever for (;;) /* infinite loop */

defines a new word, forever, for an infinite loop.

It is also possible to define macros with arguments, so the replacement text can

be different for different calls of the macro. As an example, define a macro called

max:

#define max(A, B) ((A) > (B) ? (A) : (B))

Although it looks like a function call, a use of max expands into in-line code.

Each occurrence of a formal parameter (here A or B) will be replaced by the

corresponding actual argument. Thus the line

x = max(p+q, r+sl;

will be replaced by the line

x = ((p+q) > (r+sl ? (p+q) : (r+s));

So long as the arguments are treated consistently, this macro will serve for any

90 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

data type; there is no need for different kinds of max for different data types, as

there would be with functions.

If you examine the expansion of max, you will notice some pitfalls. The

expressions are evaluated twice; this is bad if they involve side effects like incre-

ment operators or input and output. For instance,

max( i++, j++)

will increment the larger value twice. Some care also has to be taken with

parentheses to make sure the order of evaluation is preserved; consider what

happens when the macro

#define square(x) x * x

is invoked as square ( z + 1 ) .

Nonetheless, macros are valuable. One practical example comes from <stdio.

h>, in which getchar and putchar are often defined as macros to avoid the run-time

overhead of a function call per character processed. The functions in <ctype. h>

are also usually implemented as macros.

Names may be undefined with #undef, usually to ensure that a routine is

really a function, not a macro:

#undef qetchar

int qetchar(void) { ... }

Formal parameters are not replaced within quoted strings. If, however, a

parameter name is preceded by a # in the replacement text, the combination will

be expanded into a quoted string with the parameter replaced by the actual

argument. This can be combined with string concatenation to make, for exam-

ple, a debugging print macro:

#define

dprint(expr)

printf(#expr "

= %g\n", expr)

When this is invoked, as in

dprint(x/y);

the macro is expanded into

printf ( "x/y"

n :

%q\n II f

x/y) ;

and the strings are concatenated, so the effect is

printf ( "x/y

= %q\n 11 ,

x/y) ;

Within the actual argument, each

11 is replaced by \

and each \ by \\, so the

result is a legal string constant.

The preprocessor operator ## provides a way to concatenate actual argu-ments

during macro expansion. If a parameter in the replacement text is adja-cent to a

##, the parameter is replaced by the actual argument, the ## and sur-rounding

white space are removed, and the result is re-scanned. For example, the macro

paste concatenates its two arguments:

SECTION 4.11 THE C PREPROCESSOR 91

#define paste(front, back) front ## back

so paste (name, 1 ) creates the token name 1.

The rules for nested uses of ## are arcane; further details may be found in

Appendix A.

Exercise 4-14. Define a macro swap(t,x,y) that interchanges two arguments of

type t. (Block structure will help.) 0

4. 11.3 Conditional Inclusion

It is possible to control preprocessing itself with conditional statements that are

eval~ated during preprocessing. This provides a way to include code selec-tively,

depending on the value of conditions evaluated during compilation.

The #if line evaluates a constant integer expression (which may not include

sizeof, casts, or enum constants). If the expression is non-zero, subsequent lines until

an #endif or #elif or #else are included. (The preprocessor statement #elif is like else

if.) The expression defined(name) in a #if is 1 if the name has been defined, and 0

otherwise.

For example, to make sure that the contents of a file hdr. h are included only

once, the contents of the file are surrounded with a conditional like this:

#if ldefined(HDR)

#define HDR

/* contents of hdr.h go here */

#endif

The first inclusion of hdr. h defines the name HDR; subsequent inclusions will

find the name defined and skip down to the #endif. A similar style can be ~sed to

avoid including files multiple times. If this style is used consistently, then each

header can itself include any other headers on which it depends, without the user of

the header having to deal with the interdependence.

This sequence tests the name SYSTEM to decide which version of a header to

include:

#if SYSTEM == SYSV

#define HDR "sysv.h"

#eli£ SYSTEM == BSD

#define HDR "bsd.h"

#eli£ SYSTEM == MSDOS

#define HDR "msdos.h"

#else

#define HDR "default.h"

#endif

#include HDR

The #ifdef and #ifndef lines are specialized forms that test whether a

92 FUNCTIONS AND PROGRAM STRUCTURE CHAPTER 4

name is defined. The first example of #if above could have been written

#ifndef HDR

#define HDR

/* contents of hdr.h go here */

#end if

cHAPTER s: Pointers and Arrays

A pointer is a variable that contains the address of a variable. Pointers are

much used in C, partly beCause they are sometimes the only way to express a

computation, and partly because they usually lead to more compact and effi-cient

code than can be obtained in other ways. Pointers and arrays are closely related;

this chapter also explores this relationship and shows how to exploit it.

Pointers have been lumped with the goto statement as a marvelous way to

create impossible-to-understand programs. This is certainly true when they are

used carelessly, and it is easy to create pointers that point somewhere unex-

pected. With discipline, however, pointers can also be used to achieve clarity and

simplicity. This is the aspect that we will try to illustrate.

The main change in ANSI C is to make explicit the rules about how pointers

can be manipulated, in effect mandating what good programmers already prac-

tice and good compilers already enforce. In addition, the type void * (pointer to

void) replaces char *as the proper type for a generic pointer.

5. 1 Pointers and Addresses

Let us begin with a simplified picture of how memory is organized. A typi-

cal machine has an array of consecutively numbered or addressed memory cells

that may be manipulated individually or in contiguous groups. One common

situation is that any byte can be a char, a pair of one-byte cells can be treated as a

short integer, and four adjacent bytes form a long. A pointer is a group of cells

(often two or four) that can hold an address. So if cis a char and pis a pointer that

points to it, we could represent the situation this way:

The unary operator &. gives the address of an object, so the statement

94 POINTERS AND ARRAYS CHAPTER 5

P = &.c;

assigns the address of c to the variable p, and p is said to "point to" c. The &.

operator only applies to objects in memory: variables and array elements. It

cannot be applied to expressions, constants, or register variables.

The unary operator * is the indirection or dereferencing operator; when applied

to a pointer, it accesses the object the pointer points to. Suppose that x and y are

integers and ip is a pointer to int. This artificial sequence shows how to declare a

pointer and how to use &. and *:

int x = 1, y = 2, z[10];

int •ip; I• ip is a pointer to int •I

ip = &.x; I• ip now points to x •I

y = •ip; I• y is now 1 •I

•ip = 0; I• x is now 0 •I

ip = &.z[O]; I• ip now points to z[O] •I

The declarations of x, y, and z are what we've seen all along. The declaration of

the pointer ip,

int dp;

is intended as a mnemonic; it says that the expression *iP is an int. The syn-tax of

the declaration for a variable mimics the syntax of expressions in which the

variable might appear. This reasoning applies to function declarations as well.

For example,

double •dp, atof(char •);

says that in an expression *dP and atof ( s) have values of type double, and that

the argument of atof is a pointer to char.

You should also note the implication that a pointer is constrained to point to a

particular kind of object: every pointer points to a specific data type. (There is

one exception: a "pointer to void" is used to hold any type of pointer but cannot

be dereferenced itself. We'll come back to it in Section 5.11.)

If ip points to the integer x, then *iP can occur in any context where x could,

increments *iP by 10.

The unary operators * and &. bind more tightly than arithmetic operators, so

the assignment

y = •ip + 1

takes whatever ip points at, adds 1, and assigns the result to y, while

increments what ip points to, as do

SECTION 5.2 POINTERS AND FUNCTION ARGUMENTS 95

and

The parentheses are necessary in this last example; without them, the expression

would increment ip instead of what it points to, because unary operators like *

and ++ associate right to left.

Finally, since pointers are variables, they can be used without dereferencing.

For example, if iq is another pointer to int,

iq = ip

copies the contents of ip into iq, thus making iq point to whatever ip pointed to.

5. 2 Pointers and Function Arguments

Since C passes arguments to functions by value, there is no direct way for the

called function to alter a variable in the calling function. For instance, a sorting

routine might exchange two out-of-order elements with a function called swap. It

is not enough to write

swap(a, b);

where the swap function is defined as

void swap(int x, int y) /* WRONG */

{

int temp;

temp = x;

X = y;

y = temp;

}

Because of call by value, swap can't affect the arguments a and b in the rou-tine

that called it. The function above only swaps copies of a and b.

The way to obtain the desired effect is for the calling program to pass

pointers to the values to be changed:

swap( &.a, &.b);

Since the operator &. produces the address of a variable, &a is a pointer to a. In

swap itself, the parameters are declared to be pointers, and the operands are

accessed indirectly through them.

96 POINTERS AND ARRAYS CHAPTER 5

void swap(int *PX, int *PY) /* interchange *PX and *PY */

{

int temp;

temp = *PX;

*PX = *PY;

*PY = temp;

}

Pictorially:

in caller:

a:O

b:D

in swap:

.--------.

Pointer arguments enable a function to access and change objects in the

function that called it. As an example, consider a function getint that per-forms

free-format input conversion by breaking a stream of characters into integer

values, one integer per call. getint has to return the value it found and also signal

end of file when there is no more input. These values have to be passed back by

separate paths, for no matter what value is used for EOF, that could also be the

value of an input integer.

One solution is to have getint return the end of file status as its function value,

while using a pointer argument to store the converted integer back in the calling

function. This is the scheme used by scanf as well; see Section 7 .4.

The following loop fills an array with integers by calls to getint:

int n, array[SIZE], getint(int *);

for (n = 0; n <SIZE && getint(&array[n]) I= EOF; n++)

Each call sets array [ n] to the next integer found in the input and increments

n. Notice that it is essential to pass the address of array[n] to getint. Otherwise

there is no way for getint to communicate the converted integer back to the caller.

Our version of getint returns EOF for end of file, zero if the next input is not a

number, and a positive value if the input contains a valid number.

SECTION 5.3 POINTERS AND ARRAYS 97

#include <ctype.h>

int getch(void);

void ungetch(int);

/* getint: get next integer from input into *Pn */ int getint(int *Pn)

{

int c, siqn;

while (isspace(c = getch()))

/* skip white space */

if ( lisdigit(c)

ungetch(c);

&& c I= EOF

/*it's

not

c I= '+' && a

number*/

c I= ' - ') {

return 0;

}

siqn = (c == ' - ') ? -1 :

if <c == '+'II c == ' - ' )

c = getch();

for (*pn = 0; isdigit(c);

*Pn = 10 * *Pn + (c -

*Pn *= sign;

if (c I= EOF)

ungetch ( c ) ;

return c;

c = getch())

'0');

}

Throughout getint, *Pn is used as an ordinary int variable. We have also used getch

and ungetch (described in Section 4.3) so the one extra character that must be read

can be pushed back onto the input.

Exercise 5-l. As written, getint treats a + or - not followed by a digit as a valid

representation of zero. Fix it to push such a character back on the input.

Exercise 5-2. Write getfloat, the floating-point analog of getint. What type does

getfloat return as its function value? o

5. 3 Pointers and Arrays

In C, there is a strong relationship between pointers and arrays, strong enough

that pointers and arrays should be discussed simultaneously. Any opera-tion that can

be achieved by array subscripting can also be done with pointers. The pointer

version will in general be faster but, at least to the uninitiated, somewhat harder to

understand.

The declaration

98 POINTERS AND ARRAYS CHAPTER 5

int a[ 10];

defines an array a of size 10, that is, a block of 10 consecutive objects named

a[O], a[ 1], ... , a[9].

a[O]a[1]

a[9]

The notation a [ i] refers to the i-th element of the array. If pa is a pointer to an

integer, declared as

int •pa;

then the assignment

pa = &.a[O];

sets pa to point to element zero of a; that is, pa contains the address of a [ 0 ] .

Now the assignment

will copy the contents of a [ 0 ] into x.

If pa points to a particular element of an array, then by definition pa+ 1 points

to the next element, pa+i points i elements after pa, and pa-i points i

elements before. Thus, if pa points to a [ 0 ] , -

•(pa+1)

refers to the contents of a [ 1 ] , pa +i is the address of a [ i ] , and * ( pa +i ) is

the contents of a [ i ] .

These remarks are true regardless of the type or size of the variables in the

array a. The meaning of "adding 1 to a pointer," and by extension, all pointer

arithmetic, is that pa+ 1 points to the next object, and pa+i points to the i-th

SECTION 5.3 POINTERS AND ARRAYS 99

object beyond pa.

The correspondence between indexing and pointer arithmetic is very close.

By definition, the value of a variable or expression of type array is the address of

element zero of the array. Thus after the assignment

pa = &a[O];

pa and a have identical values. Since the name of an array is a synonym for the

location of the initial element, the assignment pa=&.a [ 0 ] can also be writ-ten

pa = a;

Rather more surprising, at least at first sight, is the fact that a reference to a [

i] can also be written as * ( a+i). In evaluating a [ i ], C converts it to

* ( a+i) immediately; the two forms are equivalent. Applying the operator &. to

both parts of this equivalence, it follows that &.a [ i] and a+i are also identical: a+i is

the address of the i-th element beyond a. As the other side of this coin,

if pa is a pointer, expressions may use it with a subscript; pa [ i] is identical to * (

pa+ i). In short, an array-and-index expression is equivalent to one written as a

pointer and offset.

There is one difference between an array name and a pointer that must be

kept in mind. A pointer is a variable, so pa=a and pa++ are legal. But an array

name is not a variable; constructions like a=pa and a++ are illegal.

When an array name is passed to a function, what is passed is the location of

the initial element. Within the called function, this argument is a local vari-able,

and so an array name parameter is a pointer, that is, a variable containing an

address. We can use this fact to write another version of strlen, which computes

the length of a string.

/* strlen: return length of string s */ int strlen(char *S)

{

int n;

for (n = 0; *S I= '\0'; s++)

n++;

return n;

}

Since s is a pointer, incrementing it is perfectly legal; s++ has no effect on the

character string in the function that called strlen, but merely increments strlen's

private copy of the pointer. That means that calls like

strlen( "hello, world");

string constant

strlen(array);

/*char

array[100];

strlen(ptr);

char

*Ptr; */

all work.

As formal parameters in a function definition,

100 POINTERS AND ARRAYS CHAPTERS

char s[ J;

and

char *B;

are equivalent; we prefer the latter because it says more explicitly that the

parameter is a pointer. When an array name is passed to a function, the func-tion

can at its convenience believe that it has been handed either an array or a pointer,

and manipulate it accordingly. It can even use both notations if it seems

appropriate and clear.

It is possible to pass part of an array to a function, by passing a pointer to the

beginning of the subarray. For example, if a is an array,

f(&a[2])

and

f(a+2)

both pass to the function f the address of the subarray that starts at a [ 2]. Within f,

the parameter declaration can read

f ( int arr [ J ) { ... }

f ( int *arr) { ... }

So as far as f is concerned, the fact that the parameter refers to part of a larger

array is of no consequence.

If one is sure that the elements exist, it is also possible to index backwards in

an array; p[ -1 ], p[ -2 ], and so on are syntactically legal, and refer to the ele-

ments that immediately precede p [ 0 ] . Of course, it is illegal to refer to objects

that are not within the array bounds.

5.4 Address Arithmetic

If p is a pointer to some element of an array, then p++ increments p to point to

the next element, and p+ =i increments it to point i elements beyond where it

currently does. These and similar constructions are the simplest forms of pointer

or address arithmetic.

C is consistent and regular in its approach to address arithmetic; its integra-

tion of pointers, arrays, and address arithmetic is one of the strengths of the

language. Let us illustrate by writing a rudimentary storage allocator. There are

two routines. The first, alloc ( n), returns a pointer p to n .consecutive character

positions, which can be used by the caller of alloc for storing char-acters. The

second, afree ( p), releases the storage thus acquired so it can be re-used later. The

routines are "rudimentary" because the calls to afree must be made in the

opposite order to the calls made on alloc. That' is, the storage

SECTION 5.4 ADDRESS ARITHMETIC 101

managed by alloc and afree is a stack, or last-in, first-out list. The stand-ard

library provides analogous functions called malloc and free that have no such

restrictions; in Section 8. 7 we will show how they can be implemented.

The easiest implementation is to have alloc hand out pieces of a large

character array that we will call allocbuf. This array is private to alloc and afree.

Since they deal in pointers, not array indices, no other routine need know the

name of the array, which can be declared static in the source file containing alloc

and afree, and thus be invisible outside it. In practical implementations, the array

may well not even have a name; it might instead be obtained by calling malloc or

by asking the operating system for a pointer to some unnamed block of storage.

The other information needed is how much of allocbuf has been used. We use

a pointer, called allocp, that points to the next free element. When alloc is asked

for n characters, it checks to see if there is enough room left in allocbuf. If so,

alloc returns the current value of allocp (i.e., the begin-ning of the free block),

then increments it by n to point to the next free area. If

there is no room, alloc returns zero.

afree ( p) merely sets allocp to p if

pis inside allocbuf.

before call to alloc:

allocp: \

allocbuf:

I I

--- in use -- •• ---

free

after call to alloc:

allocp: \

allocbuf:

I I

in use

free

#define ALLOCSIZE 10000 /* size of available space */

static

char

allocbuf[ALLOCSIZE];

storage

for

alloc

static

char

~allocp

= allocbuf;

free position

char *alloc(int n)

/* return pointer

to n

characters

{

if (allocbuf + ALLOCSIZE - allocp

{ /*

it fits

allocp

+= n;

return

allocp - n; /* old p */

} else

/* not enough room */

return

}

102 POINTERS AND ARRAYS CHAPTER 5

void afree(char *P} /* free storage pointed to by p */

{

if (p >= allocbuf && p < allocbuf + ALLOCSIZE} allocp = p;

}

In general a pointer can be initialized just as any other variable can, though

normally the only meaningful values are zero or an expression involving the

addresses of previously defined data of appropriate type. The declaration

static char *allocp = allocbuf;

defines allocp to be a character pointer and initializes it to point to the begin-ning

of allocbuf, which is the next free position when the program starts. This could

have also been written

static char *allocp = &allocbuf[O];

since the array name is the address of the zeroth element.

The test

if (allocbuf + ALLOCSIZE - allocp >= n} { /* it fits */

checks if there's enough room to satisfy a request for n characters. If there is, the

new value of allocp would be at most one beyond the end of allocbuf. If the

request can be satisfied, alloc returns a pointer to the beginning of a block of

characters (notice the declaration of the function itself). If not, alloc must return

some signal that no space is left. C guarantees that zero is n~ver a valid address for

data, so a return value of zero can be used to signal an abnor-mal event, in this case,

no space.

Pointers and integers are not interchangeable. Zero is the sole exception: the

constant zero may be assigned to a pointer, and a pointer may be compared with

the constant zero. The symbolic constant NULL is often used in place of zero, as

a mnemonic to indicate more clearly that this is a special value for a pointer.

NULL is defined in <stdio. h>. We will use NULL henceforth.

Tests like

if (allocbuf + ALLOCSIZE - allocp >= n} { /* it fits */

and

if (p >= allocbuf && p < allocbuf + ALLOCSIZE}

show several important facets of pointer arithmetic. First, pointers may be com-

pared under certain circumstances. If p and q point to members of the same

array, then relations like ==, I=, <, >=, etc., work properly. For example,

p < q

is true if p points to an earlier member of the array than q does. Any pointer can

be meaningfully compared for equality or inequality with zero. But the behavior

is undefined for arithmetic or comparisons with pointers that do not

SECTION 5.4 ADDRESS ARITHMETIC 103

point to members of the same array. (There is one exception: the address of the

first element past the end of an array can be used in pointer arithmetic.)

Second, we have already observed that a pointer and an integer may be added

or subtracted. The construction

P + n

means the address of the n-th object beyond the one p currently points to. This is

true regardless of the kind of object p points to; n is scaled according to the size

of the objects p points to, which is determined by the declaration of p. If an intis

four bytes, for example, the int will be scaled by four.

Pointer subtraction is also valid: if p and q point to elements of the same

array, and p<q, then q-p+ 1 is the number of elements from p to q inclusive. This

fact can be used to write yet another version of strlen:

/* strlen: return length of string s */ int strlen(char *S)

{

char *P = s;

while (*P I= '\0')

p++;

return p - s;

}

In its declaration, p is initialized to s, that is, to point to the first character of the

string. In the while loop, each character in turn is examined until the ''\0' at the

end is seen. Because p points to characters, p++ advances p to the next character

each time, and p-s give~ the number of characters advanced over, that is, the string

length. (The number of characters in the string could be too large to store in an

int. The header <stddef. h> defines a type ptrdiff _ t that is large enough to hold

the signed difference of two pointer values. If we were being very cautious,

however, we would use size_ t for the return type of strlen, to match the standard

library version. size_ t is the unsigned integer type returned by the sizeof

operator.)

Pointer arithmetic is consistent: if we had been dealing with floats, which

occupy more storage than chars, and if p were a pointer to float, p++ would

advance to the next float. Thus we could write another version of alloc that

maintains floats instead of chars, merely by changing char to float throughout

alloc and afree. All the pointer manipulations automatically take into account the

size of the object pointed to.

The valid pointer operations are assignment of pointers of the same type,

adding or subtracting a pointer and an integer, subtracting or comparing two

pointers to members of the same array, and assigning or comparing to zero. All

other pointer arithmetic is illegal. It is not legal to add two pointers, or to mul-

tiply or divide or shift or mask them, or to add float or double to them, or even,

except for void *• to assign a pointer of one type to a pointer of another type

without a cast.

104 POINTERS AND ARRAYS CHAPTER 5

5.5 Character Pointers and Functions

A string constant, written as

"I am a string"

is an array of characters. In the internal representation, the array is terminated

with the null character '\0' so that programs can find the end. The length in

storage is thus one more than the number of characters between the double

quotes.

Perhaps the most common occurrence of string constants is as arguments to

functions, as in

printf( "hello, world\n");

When a character string like this appears in a program, access to it is through a

character pointer; print£ receives a pointer to the beginning of the character

array. That is, a string constant is accessed by a pointer to its first element.

String constants need not be function arguments. If pmessage is declared

char *pmessage;

then the statement

pmessage = "now is the time";

assigns to pmessage a pointer to the character array. This is not a string copy;

only pointers are involved. C does not provide any operators for process-ing an

entire string of characters as a unit.

There is an important difference between these definitions:

char amessage[]

char *pmessage

= "now is the time"; "now

is the time";

/* an array */ /* a

pointer */

amessage is an array, just big enough to hold the sequence of characters and '\0'

that initializes it. Individual characters within the array may be changed but

amessage will always refer to the same storage. On the other hand, pmessage is a

pointer, initialized to point to a string constant; the pointer may subsequently be

modified to point elsewhere, but the result is undefined if you try to modify the

string contents.

pmessage: c:Jr---...,~~ now is the time\0 I I I

We will illustrate more aspects of pointers and arrays by studying versions of

two useful functions adapted from the standard library. The first function is

strcpy ( s , t), which copies the string t to the string s. It would be nice just to say

s=t but this copies the pointer, not the characters. To copy the

SECTION 5.5 CHARACTER POINTERS AND FUNCTIONS 105

characters, we need a loop. The array version is first:

I* strcpy: copy t to s; array subscript version */ void strcpy(char *S, char *t)

{

int i;

i = 0;

while ( (s[i) = t[i)) I= '\0')

i++;

}

For contrast, here is a version of strcpy with pointers:

I* strcpy: copy t to s; pointer version 1 */ void strcpy(char *S, char

*t)

{

while ( (*s = *t) I= '\0') {

S++;

t++;

}

Because arguments are passed by value, strcpy can use the parameters s and

t in any way it pleases. Here they are conveniently initialized pointers, which

are marched along the arrays a character at a time, until the ''\0' that ter-minates t

has been copied to s.

In practice, strcpy would not be written as we showed it above. Experi-enced

C programmers would prefer

I* strcpy: copy t to s; pointer version 2 */ void strcpy(char *S, char

*t)

{

while ( (*S++ = *t++) I= '\0')

}

This moves the increment of s and t into the test part of the loop. The value of

*t++ is the character that t pointed to before t was incremented; the postfix

++ doesn't change t until after this character has been fetched. In the same way,

the character is stored into the old s position before s is incremented. This

character is also the value that is compared against ''\0' to control the loop. The

net effect is that characters are copied from t to s, up to and includ-ing the

terminating ''\0 '.

As the final abbreviation, observe that a comparison against '\0' is redun-dant,

since the question is merely whether the expression is zero. So the func-tion

would likely be written as

106 POINTERS AND ARRAYS CHAPTER 5

I• strcpy: copy t to s; pointer version 3 •I void strcpy(char •s,

char •t)

{

while (•s++ = •t++)

}

Although this may seem cryptic at first sight, the notational convenience is con-

siderable, and the idiom should be mastered, because you will see it frequently

inC programs.

The strcpy in the standard library (<string. h>) returns the target string as its

function value.

The second routine that we will examine is strcmp ( s , t), which compares the

character strings s and t, and returns negative, zero or positive if s is lexi-

cographically less than, equal to, or greater than t. The value is obtained by

subtracting the characters at the first position where s and t disagree.

I• strcmp: return <0 if s<t, 0 if s==t, >0 if s>t •I int strcmp(char •s, char

•t)

{

int i;

for (i = 0; s[i] == t[i]; i++)

if (s[i] == '\0')

return 0;

return s[i] - t[i];

}

The pointer version of strcmp:

I• strcmp: return <0 if s<t, 0 if s==t, >0 if s>t •I int strcmp(char •s, char

•t)

{

for ( ; •s == •t; s++, t++)

if (•s == '\0')

return 0;

return •s - •t;

}

Since ++ and -- are either prefix or postfix operators, other combinations of

* and ++ and -- occur, although less frequently. For example,

decrements p before fetching the character that p points to. In fact, the pair of

expressions

*P++ = val; I• push val onto stack •I

val = •--p; I• pop top of stack into val •I

are the standard idioms for pushing and popping a stack; see Section 4.3.

The header <string. h> contains declarations for the functions mentioned

SECTION 5.6 POINTER ARRAYS; POINTERS TO POINTERS 107

in this section, plus a variety of other string-handling functions from the stand-

ard library.

Exercise 5-3. Write a pointer version of the function strcat that we showed in

Chapter 2: strcat ( s, t) copies the string t to the end of s. D

Exercise 5-4. Write the function strend ( s, t), which returns 1 if the string

t occurs at the end of the string s, and zero otherwise. D

Exercise 5-5. Write versions of the library functions strncpy, strncat, and strncmp;

which operate on at most the first n characters of their argument strings. For

example, strncpy ( s, t, n) copies at most n characters of t to s. Full descriptions are in

Appendix B. D

Exercise 5-6. Rewrite appropriate programs from earlier chapters and exercises

with pointers instead of array indexing. Good possibilities include getline

(Chapters 1 and 4), atoi, i toa, and their variants (~hapters 2, 3, and 4), reverse

(Chapter 3), and strindex and getop (Chapter 4). D

5.6 Pointer Arrays; Pointers to Pointers

Since pointers are variables themselves, they can be stored in arrays just as

other variables can. Let us illustrate by writing a program that will sort a set of

text lines into alphabetic order, a stripped-down version of the UNIX program

sort.

In Chapter 3 we presented a Shell sort function that would sort an array of

integers, and in Chapter 4 we improved on it with a quicksort. The same algo-

rithms will work, except that now we have to deal with lines of text, which are of

different lengths, and which, unlike integers, can't be compared or moved in a

single operation. We need a data representation that will cope efficiently and

conveniently with variable-length text lines.

This is where the array of pointers enters. If the lines to be sorted are stored

end-to-end in one long character array, then each line can be accessed by a

pointer to its first character. The pointers themselves can be stored in an array.

Two lines can be compared by passing their pointers to strcmp. When two out-of-

order lines have to be .exchanged, the pointers in the pointer array are

exchanged, not the text lines themselves.

This eliminates the twin problems of complicated storage management and high

overhead that would go with moving the lines themselves.

108 POINTERS AND ARRAYS CHAPTER 5

The sorting process has three steps:

read all the lines of input

sort them

print them in order

As usual, it's best to divide the program into functions that match this natural

division, with the main routine controlling the other functions. Let us defer the

sorting step for a moment, and concentrate on the data structure and the input and

output.

The input routine has to collect and save the characters of each line, and build

an array of pointers to the lines. It will also have to count the number of input

lines, since that information is needed for sorting and printing. Since the input

function can only cope with a finite number of input lines, it can return some

illegal line count like -1 if too much input is presented.

The output routine only has to print the lines in the order in which they appear

in the array of pointers.

#include <stdio.h>

#include <string.h>

#define MAXLINES 5000 /* max #lines to be sorted */

char *lineptr[MAXLINES]; I* pointers to text lines */

int readlines(char *lineptr[], int nlines); void writelines(char

*lineptr[], int nlines);

void qsort(char *lineptr[], int left, int right);

/* sort input lines */

main()

{

int nlines; I* number of input lines read */

if ((nlines = readlines(lineptr, MAXLINES)) >• 0) { qsort(lineptr, 0,

nlines-1); writelines(lineptr, nlines);

return 0;

} else {

printf("error: input too big to sort\n"); return 1;

}

SECTION 5.6 POINTER ARRAYS; POINTERS TO POINTERS 109

#define MAXLEN 1000 I• max length of any

input line •I

int getline(char *• int);

char •alloc(int);

I• readlines: read input lines •I

int readlines(char •lineptr[l. int maxlines)

{

int len. nlines;

char *Pt line[MAXLEN];

nlines = 0;

while ((len= qetline(line. MAXLEN)) > 0)

if (nlines >= maxlines :: (p = alloc(len)) --NULL)

return -1;

else {

line[len-1] = '\0'; I• delete newline •I

strcpy ( p. line ) ;

lineptr[nlines++] = p;

}

return nlines;

}

I• writelines: write output lines •I

void writelines(char •lineptr[]. int nlines)

{

int i;

for (i = 0; i < nlines; i++)

printf( ""s\n" • lineptr[i]);

}

The function getline is from Section 1.9.

The main new thing is the declaration for lineptr:

char •lineptr[MAXLINES]

says that lineptr is an array of MAXLINES elements, each element of which is a

pointer to a char. That is, lineptr [ i] is a character pointer, and *lineptr [ i] is the

character it points to, the first character of the i-th saved te:lt line.

Since lineptr is itself the name of an array, it can be treated as a pointer in the

same manner as in our earlier examples, and wri telines can be writ-ten instead as

I• writelines: write output lines •I

void writelines(char •lineptr[], int nlines)

{

while (nlines-- > 0)

printf ( ""s\n", •lineptr++);

}

110 POINTERS AND ARRAYS CHAPTERS

Initially *lineptr points to the first line; each increment advances it to the next line

pointer while nlines is counted down.

With input and output under control, we can proceed to sorting. The quick-sort

from Chapter 4 needs minor changes: the declarations have to be modified, and the

comparison operation must be done by calling strcmp. The algorithm remains the

same, which gives us some confidence that it will still work.

I* qsort: sort v[left] ... v[right] into increasing order*/ void qsort(char *V[], int left, int

right)

{

int

last;

void swap(char *V[], inti, int j);

if (left >= right)

/* do nothing if array contains */

return;

I* fewer

than two elements */

swap(v,

left,

(left

right)/2);

last

left;

for

= ~eft+1;

i <=

right;

i++)

if (strcmp(v[i],

v[left])

< 0)

swap(v,

++last, i);

swap(v,

left,

last);

qsort(v, left, last-1);

qsort(v, last+1,

right);

}

Similarly, the swap routine needs only trivial changes:

/* swap: interchange v[i] and v[j] */ void swap(char

*V[], inti, int j)

{

char *temp;

temp= v[i];

v[i] = v[j];

v[j] = temp;

}

Since any individual element of v (alias lineptr) is a character pointer, temp must be

also, so one can be copied to the other.

Exercise 5-7. Rewrite readlines to store lines in an array supplied by main, rather than

calling alloc to maintain storage. How much faster is the pro-gram? D

5. 7 Multi-dimensional Arrays

C provides rectangular multi-dimensional arrays, although in practice they are

much less used than arrays of pointers. In this section, we will show some of their

properties.

SECTION 5.7 MULTI-DIMENSIONAL ARRAYS Ill

Consider the problem of date conversion, from day of the month to day of the

year and vice versa. For example, March 1 is the 60th day of a non-leap year, and

the 61st day of a leap year. Let us define two functions to do the conversions:

day_of_year converts the month and day into the day of the year, and month_day

converts the day of the year into the month and day. Since this latter function

computes two values, the month and day arguments will be pointers:

month_day(1988, 60, &m, &d)

sets m to 2 and d to 29 (February 29th).

These functions both need the same information, a table of the number of

days in each month ("thirty days hath September ... "). Since the number of days

per month differs for leap years and non-leap years, it's easier to separate them

into two rows of a two-dimensional array than to keep track of what hap-pens to

February during computation. The array and the functions for perform-ing the

transformations are as follows:

static char daytab[2][13] = {

{0,

31,

28,

31,

30,

31,

30,

31,

30,

31,

30,

31},

{0,

31,

29,

31,

30,

31,

30,

31,

30,

31,

30,

31}

} ;

/* day_of_year: set day of year from month & day */ int

day_of_year(int year, int month, int day)

{

int i, leap;

leap = year%4 == 0 && year%100 I= 0 :: year%400 == 0; for (i = 1; i <

month; i++)

day += daytab[leap][i];

return day;

}

/* month_day: set month, day from day of year */

void month_day(int year, int yearday, int *pmonth, int *Pday)

{

int i, leap;

leap = year%4 == 0 && year%100 I= 0 :: year%400 -- 0; for (i = 1;

yearday > daytab[leap][i]; i++)

yearday -= daytab[leap][i];

*pmonth = i;

*Pday = yearday;

}

Recall that the arithmetic value of a logical expression, such as the one for leap,

is either zero (false) or one (true), so it can be used as a subscript of the array daytab.

The array daytab has to be external to both day_ of _year and

112 POINTERS AND ARRAYS CHAPTER 5

month_day, so they can both use it. We made it char to illustrate a legiti-mate

use of char for storing small non-character integers.

daytab is the first two-dimensional array we have dealt with. In C, a two-

dimensional array is really a one-dimensional array, each of whose elements is

an array. Hence subscripts are written as

daytab[i)[j] /* [row][col] */

rather than

daytab[i,j]

Other than this notational distinction, a two-dimensional array can be treated in

much the same way as in other languages. Elements are stored by rows, so the

rightmost subscript, or column, varies fastest as elements are accessed in storage

order.

An array is initialized by a list of initializers in braces; each row of a two-

dimensional array is initialized by a corresponding sub-list. We started the array

daytab with a column of zero so that month numbers can run from the natural 1

to 12 instead of 0 to 11. Since space is not at a premium here, this is clearer than

adjusting the indices.

If a two-dimensional array is to be passed to a function, the parameter declaration

in the function must include the number of columns; the number of rows is

irrelevant, since what is passed is, as before, a pointer to an array of rows, where

each row is an array of 13 ints. In this particular case, it is a pointer to objects that

are arrays of 13 ints. Thus if the array daytab is to be passed to a function f, the

declaration off would be

f ( int daytab [ 2 ][ 13 ] ) { .. . }

It could also be

f ( int daytab [ ] [ 13 ] ) { .. . }

since the number of rows is irrelevant, or it could be

f ( int ( *daytab )[ 13 ] ) { .. . }

which says that the parameter is a pointer to an array of 13 integers. The

parentheses are necessary since brackets [] have higher precedence than *·

Without parentheses, the declaration

int *daytab[13]

is an array of 13 pointers to integers. More generally, only the first dimension

(subscript) of an array is free; all the others have to be specified.

Section 5.12 has a further discussion of complicated declarations.

Exercise 5·8. There is no error checking in day_ of _year or month_ day.

Remedy this defect. D

SECTION 5.9 POINTERS VS. MULTI-DIMENSIONAL ARRAYS 113

5.8 Initialization of Pointer Arrays

Consider the problem of writing a function month_name ( n), which returns a

pointer to a character string containing the name of the n-th month. This is an ideal

application for an internal static array. month_name contains a private array of

character strings, and returns a pointer to the proper one when called. This section

shows how that array of names is initialized.

The syntax is similar to previous initializations:

/* month_name: return name of n-th month */ char

*month_name(int n)

{

static char *name(] = {

"Illegal month",

"January", "February", "March", "April",

"May", "June", "July", "August", "September",

"October", "November", "December"

} ;

return ( n < 1 l l n > 12 ) _? name ( 0 ] : name ( n] ;

}

The declaration of name, which is an array of character pointers, is the same as 1

ineptr in the sorting example. The initializer is a list of character strings; each is

assigned to the corresponding position in the array. The characters of the i-th string

are placed somewhere, and a pointer to them is stored in name [ i ] . Since the size of

the array name is not specified, the compiler counts the initializers and fills in the

correct number.

5.9 Pointers vs. Multi-dimensional Arrays

Newcomers to C are sometimes confused about the difference between a two-

dimensional array and an array of pointers, such as name in the example above.

Given the definitions

int a( 10] (20];

int *b(10];

then a [ 3 ] [ 4] and b [ 3 ] [ 4] are both syntactically legal references to a single int.

But a is a true two-dimensional array: 200 int-sized locations have been set aside,

and the conventional rectangular subscript calculation 20Xrow+col is used to find the

element al[row] [co/]. Forb, however, the definition only allo-cates 10 pointers and

does not initialize them; initialization must be done expli-citly, either statically or

with code. Assuming that each element of b does point to a twenty-element array,

then there will be 200 ints set aside, plus ten cells for the pointers. The important

advantage of the pointer array is that the rows of the array may be of different

lengths. That is, each element of b need not

114 POINTERS AND ARRAYS CHAPTER 5

point to a twenty-element vector; some may point to two elements, some to fifty,

and some to none at all.

Although we have phrased this discussion in terms of integers, by far the

most frequent use of arrays of pointers is to store character strings of diverse

lengths, as in the function month_name. Compare the declaration and picture for an

array of pointers:

char *name[] = { "Illegal month",

"Jan, "Feb", "Mar" } ;

name:

.------

Illegal

month\ol

Jan,ol

Feb,ol

-----;~ Mar\o I

with those for a two-dimensional array:

char aname[][15] = { "Illegal month", "Jan", "Feb", "Mar" };

aname:

!Illegal

month\o Jan\o

Feb\o

Mar\o

Exercise 5-9.

Rewrite the

routines day_of_year

and month_ day with

pointers instead of indexing.

5.10 Command-line Arguments

In environments that support C, there is a way to pass command-line argu-

ments or parameters to a program when it begins executing. When main is called,

it is called with two arguments. The first (conventionally called argc, for

argument count) is the number of command-line arguments the program was

invoked with; the second (argv, for argument vector) is a pointer to an array of

character strings that contain the arguments, one per string. We cus-tomarily use

multiple levels of pointers to manipulate these character strings.

The simplest illustration is the program echo, which echoes its command-line

arguments on a single line, separated by blanks. That is, the command

echo hello, world

prints the output

hello, world

SECTION 5.10 COMMAND-LINE ARGUMENTS 115

By convention, argv[ 0] is the name by which the program was invoked, so argc is at

least 1. If argc is l, there are no command-line arguments after the program name. In

the example above, argc is 3, and argv[ 0 ], argv[ 1 ], and argv[2] are "echo", "hello,",

and "world" respectively. The first optional argument is argv[ 1] and the last is

argv[argc-1 ]; additionally, the standard requires that argv[argc] be a null pointer.

echo\ a

world\o

The first version of echo treats argv as an array of character pointers:

#include <stdio.h>

/* echo command-line arguments; 1st version */ main(int argc,

char *argv[])

{

int

for

= 1; i < argc; i++)

print£( "%s%s 11 , argv[i], (i < argc-1) ? 11 11

1111

);

print£(

);

return

}

Since argv is a pointer to an array of pointers, we can manipulate the pointer rather

than index the array. This next variation is based on incrementing argv, which is a

pointer to pointer to char, while argc is counted down:

#include <stdio.h>

I* echo command-line arguments;

2nd version

main(int argc, char *argv[])

{

while (--argc > 0)

print£ ( 11 %s%s 11 , *++argv,

( argc > 1) ?

1111

) ;

print£ (

) ;

return 0;

}

Since argv is a pointer to the beginning of the array of argument strings, incre-

menting it by 1 ( + +argv) makes it point at the original argv [ 1 ] instead of argv [ 0 ] .

Each successive increment moves it along to the next argument; *argv is then the

pointer to that argument. At the same time, argc is decre-mented; when it becomes

zero, there are no arguments left to print.

Alternatively, we could write the printf statement as

116

POINTERS AND ARRAYS

CHAPTERS

printf((argc > 1)?

11 "s

"811

*++argv);

This shows that the format argument of printf can be an expression too.

As a second example, let us make some enhancements to the pattern-finding

program from Section 4.1. If you recall, we wired the search pattern deep into the

program, an obviously unsatisfactory arrangement. Following the lead of the

UNIX program grep, let us change the program so the pattern to be matched is

specified by the first argument on the command line.

#include <stdio.h>

#include <string.h>

#define MAXLINE 1000

int getline(char *line, int max);

I* find: print lines that match pattern from 1st arg */ main(int argc, char

*argv[])

{

char line[MAXLINE];

int found = 0;

if (argc I= 2)

printf( 11 Usage: find pattern\n 11 );

else

while (getline(line, MAXLINE) > 0)

if (strstr(line, argv[1)) I= NULL) {

print£ ( 11 "s 11 , l,ine) ;

found++;

}

return found;

}

The standard library function strstr ( s , t)

occurrence of the string t in the string s, or

returns

NULL

a pointer to the if

there is none.

first

It is

declared in <string. h>.

The model can now be elaborated to illustrate further pointer constructions.

Suppose we want to allow two optional arguments. One says "print all lines

except those that match the pattern;" the second says "precede each printed line by

its line number."

A common convention for C programs on UNIX systems is that an argument

that begins with a minus sign introduces an optional flag or parameter. If we

choose -x (for "except") to signal the inversion, and -n ("number") to request line

numbering, then the command

find -x -n pattern

will print each line that doesn't match the pattern, preceded by its line number.

Optional arguments should be permitted in any order, and the rest of the

program should be independent of the number of arguments that were present.

Furthermore, it is convenient for users if option arguments can be combined, as

SECTION 5.10 COMMAND-LINE ARGUMENTS 117

find -nx pattern

Here is the program:

#include <stdio.h>

#include <string.h>

#define MAXLINE 1000

int getline(char *line, int max);

/* find: print lines that match pattern from 1st arg */ main(int argc, char

*argv[])

{

char line[MAXLINE];

long lineno = 0;

int c, except = 0, number = 0, found = 0;

while (--argc > 0 && (*++argv)[O] == '-')

while (c = *++argv[O])

switch (c) {

case

'x, :

except =

1 .

break;

case 'n, :

number =

break;

default:

printf("find: illegal option "c\n 11 , c);

argc = 0;

found = -1;

break;

}

if (argc I= 1)

printf("Usage: find -x -n pattern\n");

else

while (getline(line, MAXLINE) > 0) {

lineno++;

if ((strstr(line, *argv) I=

NULL) I= except) {

if (number)

print£ ( 11 "ld: 11 , lineno) ;

print£ ( ""s 11 , line) ;

found++;

}

return found;

}

argc is decremented and argv is incremented before each optional argu-ment. At

the end of the loop, if there are no errors, argc tells how many argu-ments remain

unprocessed and argv points to the first of these. Thus argc

118 POINTERS AND ARRAYS CHAPTER 5

should be 1 and *argv should point at the pattern. Notice that *++argv is a pointer

to an argument string, so ( *++argv) [ 0] is its first character. (An alternate valid

form would be **++argv.) Because [] binds tighter than * and ++, the

parentheses are necessary; without them the expression would be taken as *++ (

argv[ 0]). In fact, that is what we used in the inner loop, where the task is to walk

along a specific argument string. In the inner loop, the expression *++argv[ 0]

increments the pointer argv[ 0 ]!

It is rare that one uses pointer· expressions more complicated than these; in such

cases, breaking them into two or three steps will be more intuitive.

Exercise 5-10. Write the program expr, which evaluates a reverse Polish

expression from the command line, where each operator or operand is a separate

argument. For example,

~xpr 2 3 4 + *

evaluates 2 x (3+4). D

Exercise 5-11. Modify the programs entab and detab (written as exercises in

Chapter 1) to accept a list of tab stops as arguments. Use the default tab set-tings

if there are no arguments. D

Exercise 5-12. Extend entab and detab to accept the shorthand

entab -m +n

to mean tab stops every n columns, starting at column m. Choose convenient (for

the user) default behavior. D

Exercise 5-13. Write the program tail, which prints the last n lines of its input. By

default, n is 10, let us say, but it can be changed by an optional argument, so that

tail -n

prints the last n lines. The program should behave rationally no matter how

unreasonable the input or the value of n. Write the program so it makes the best

use of available storage; lines should be stored as in the sorting program of

Section 5.6, not in a two-dimensional array of fixed size. D

5.11 Pointers to Functions

In C, a function itself is not a variable, but it is possible to define pointers to

functions, which can be assigned, placed in arrays, passed to functions, returned

by functions, and so on. We will illustrate this by modifying the sorting pro-

cedure written earlier in this chapter so that if the optional argument -n is given,

it will sort the input lines numerically instead of lexicographically.

A sort often consists of three parts-a comparison that determines the

SECTION 5.11 POINTERS TO FUNCTIONS 119

ordering of any pair of objects, an exchange that reverses their order, and a

sorting algorithm that makes comparisons and exchanges until the objects are in

order. The sorting algorithm is independent of the comparison and exchange

operations, so by passing different comparison and exchange functions to it, we

can arrange to sort by different criteria. This is the approach taken in our new

sort.

Lexicographic comparison of two lines is done by strcmp, as before; we will

also need a routine numcmp that compares two lines on the basis of numeric

value and returns the same kind of condition indication as strcmp does. These

functions are declared ahead of main and a pointer to the appropriate one is

passed to qsort. We have skimped on error processing for arguments, so as to

concentrate on the main issues.

#include <stdio.h>

#include <string.h>

#define MAXLINES 5000 /* max #lines to be sorted */ char

*lineptr[MAXLINES]; /*pointers to text lines */

int readlines(char *lineptr[], int nlines); void writelines(char

*lineptr[], int nlines);

void qsort(void *lineptr[], int left, int right, int (*comp)(void *•

void*));

int numcmp(char *• char*);

/* sort input lines */

main(int argc, char *arqv[])

{

int nlines;

int numeric = 0;

number of input lines read */ 1 if

numeric sort */

if (argc > 1 &.&.

strcmp(arqv[1],

"-n") == 0)

numeric = 1;

if ((nlines = readlines(lineptr, MAXLINES)) >= 0) { qsort((void **)

lineptr, 0, nlines-1,

(int (*)(void*,void*))(numeric ? numcmp : strcmp));

writelines(lineptr, nlines);

return 0;

} else {

printf ( "input too big to sort\n" ) ; return 1;

}

In the call to qsort, strcmp and numcmp are addresses of functions. Since they are

known to be functions, the &. operator is not necessary, in the same way that it is

not needed before an array name.

We have written qsort so it can process any data type, not just character

120 POINTERS AND ARRAYS CHAPTER 5

strings. As indicated by the function prototype, qsort expects an array of

pointers, two integers, and a function with two pointer arguments. The generic

pointer type void *is used for the pointer arguments. Any pointer can be cast to

void * and back again without loss of information, so we can call qsort by casting

arguments to void *· The elaborate cast of the function argument casts the

arguments of the comparison function. These will generally have no effect on

actual representation, but assure the compiler that all is well.

/* qsort: sort v[left] •.. v[right] into increasing order*/ void qsort(void *V[], int

left, int right,

int (*comp)(void *• void*))

{

int i, last;

void swap(void *V[], int, int);

if (left >= right) /* do nothing if array contains */

return; /* fewer than two elements */

swap(v, left, (left + right)/2);

last = left;

for (i = left+1; i <= right; i++)

if ((*comp)(v[i], v[left]) < 0)

swap(v, ++last, i);

swap(v, left, last);

qsort(v, left, last-1, comp);

qsort(v, last+1, right, comp);

}

The declarations should be studied with some care. The fourth parameter of qsort

int (*comp)(void *• void*)

which says that comp is a pointer to a function that has two void *arguments and

returns an int.

The use of comp in the line

if ((*comp)(v(i], v[left]) < 0)

is consistent with the declaration: comp is a pointer to a function, *comp is the

function, and

(*COmp)(v[i], v[left])

is the call to it. The parentheses are needed so the components are correctly

associated; without them,

int *COmp(void *• void *) I* WRONG */

says that comp is a function returning a pointer to an int, which is very dif-ferent.

We have already shown strcmp, which compares two strings. Here is

numcmp, which compares two strings on a leading numeric value, computed by

SECTION 5.11 POINTERS TO FUNCTIONS 121

calling atof:

#include <stdlib.h>

I* numcmp: compare s1 and s2 numerically */ int

numcmp(char *S1, char *S2)

{

double v1, v2;

v1 = atof(s1);

v2 = atof( s2);

if (v1 < v2)

return -1;

else if (v1 > v2)

return

1 .

else

return 0;

}

The swap function, which exchanges two pointers, is identical to what we

presented earlier in the chapter, except that the declarations are changed to

void*·

void swap(void *V[], int i, int j)

{

void *temp;

temp = v[i];

v[i] = v[j];

v[j] = temp;

}

A variety of other options can be added to the sorting program; some make

challenging exercises.

Exercise 5-14. Modify the sort program to handle a -r flag, which indicates

sorting in reverse (decreasing) order. Be sure that -r works with -n. 0

Exercise 5-15. Add the option -f to fold upper and lower case together, so that

case distinctions are not made during sorting; for example, a and A compare

equal. o

Exercise 5-16. Add the -d ("directory order") option, which makes comparis-ons

only on letters, numbers and blanks. Make sure it works in conjunction with -f. 0

Exercise 5-17. Add a field-handling capability, so sorting may be done on fields

within lines, each field sorted according to an independent set of options. (The

index for this book was sorted with -df for the index category and -n for the page

numbers.) o

122 POINTERS AND ARRAYS CHAPTER 5

5. 12 Complicated Declarations

C is sometimes castigated for the syntax of its declarations, particularly ones

that involve pointers to functions. The syntax is an attempt to make the

declaration and the use agree; it works well for simple cases, but it can be

confusing for the harder ones, because declarations cannot be read left to right,

and because parentheses are over-used. The difference between

int *f(); /* f: function returning pointer to int */

and

int ( *Pf ) ( ) ; /* pf: pointer to function returning int */

illustrates the problem: * is a prefix operator and it has lower precedence than ( )

, so parentheses are necessary to force the proper association.

Although truly complicated declarations rarely arise in practice, it is impor-

tant to know how to understand them, and, if necessary, how to create them. One

good way to synthesize declarations is in small steps with typedef, which is

discussed in Section 6. 7. As an alternative, in this section we will present a pair

of programs that convert from valid C to a word description and back again. The

word description reads left to right.

The first, del, is the more complex. It converts a C declaration into a word

description, as in these examples:

char **argv

argv: pointer to pointer to char

int (*daytab)[13]

daytab: pointer to array[13] of int

int *daytab[13]

daytab: array[13] of pointer to int

void *COmp ( )

comp: function returning pointer to void

void (*comp)()

comp: pointer to function returning void

char ( * (*X ( ) ) [ ] ) ( )

x: function returning pointer to array[] of

pointer to function returning char

char (*(*X[3])())[5]

x: array[3] of pointer to function returning pointer to

array[S] of char

del is based on the grammar that specifies a declarator, which is spelled out

precisely in Appendix A, Section 8.5; this is a simplified form:

direct -del:

name

(del)

direct -del ( )

direct -del [optional size 1

In words, a

del

is a

direct-de/,

perhaps preceded by *'s.

direct-de/

is a

SECTION 5.12 COMPLICATED DECLARATIONS 123

name, or a parenthesized del, or a direct-de/ followed by parentheses, or a direct-

de/ followed by brackets with an optional size.

This grammar can be used to parse declarations. For instance, consider this

declarator:

(*pfa[))()

pf a will be identified as a name and thus as a direct -del. Then pfa [ ] is also

a direct -del. Then *Pf a [ ] is a recognized as a del, so ( *Pfa [ ] ) is a direct-del.

Then ( *Pfa[]) () is a direct-icl and thus a del. We can also illustrate

the parse with a parse tree like this (where direct-de/ has been abbreviated to

dir-dc/): .

pfa

[ ]

( )

name

dir-del

del

dir-del

del

The heart of the del program is a pair of functions, del and dirdel, that parse a

declaration according to this grammar. Because the grammar is recur-sively

defined, the functions call each other recursively as they recognize pieces of a

declaration; the program is called a recursive-descent parser.

/* del: parse a declarator */

void dcl(void)

{

int ns;

for (ns = 0; gettoken() == '*';

ns++;

dirdcl();

while (ns-- > 0)

strcat(out, " pointer to");

count *'s */

}

124 POINTERS AND ARRAYS CHAPTERS

I• dirdcl: parse a direct declarator •I void dirdcl(void)

{

int type;

if (tokentype == '(') {

I• ( del )

•I

del ();

if (tokentype I= ')')

printf("error: missing

)\n");

} else if (tokentype == NAME)

I•

variable name

•I

strcpy(name, token);

else

printf("error: expected name or (dcl)\n");

while ((type=gettoken()) ==PARENS I I type== BRACKETS)

if (type == PARENS)

strcat(out, "function returning"); else {

strcat(out, "array");

strcat(out, token);

strcat(out, " of");

}

Since the programs are intended to be illustrative, not bullet-proof, there are

significant restrictions on del. It can only handle a simple data type like char or

int. It does not handle argument types in functions, or qualifiers like const.

Spurious blanks confuse it. It doesn't do much error recovery, so invalid declarations

will also confuse it. These improvements are left as exer-cises.

Here are the global variables and the main routine:

#include <stdio.h>

#include <string.h>

#include <ctype.h>

#define MAXTOKEN100

enum { NAME, PARENS, BRACKETS } ;

void

dcl(void);

void

dirdcl(void);

int

gettoken(void);

I• type of last token •I

int

tokentype;

char

token [ MAXTOKEN] ;

I• last token string •I

char name [ MAXTOKEN] ;

I• identifier name •I

char datatype[MAXTOKEN];

I• data type = char, int, etc. •I

char out[ 1000];

I• output string •I

SECTION 5.12 COMPLICATED DECLARATIONS 125

main() /*convert declaration to words*/

{

while (gettoken() I= EOF) { /*1st token on line*/

strcpy(datatype, token); /*is the datatype */ out[O] = '\.0';

del(); /*parse rest of line*/

if (tokentype I= '\.n')

printf("syntax error\.n");

printf ( ""s: "s "s\.n", name, out, datatype) ;

}

return 0;

}

The function gettoken skips blanks and tabs, then finds the next token in the

input; a "token" is a name, a pair of parentheses, a pair of brackets perhaps

including a number, or any other single character.

int gettoken(void) /* return next token */

{

int c, getch(void);

void ungetch(int);

char *P = token;

while ( ( c = getch()) == ' ' II c == '\.t')

if ( c == , ( , ) {

if ((c = getch()) == ')') {

strcpy(token, "()");

return tokentype = PARENS;

} else { ungetch ( c

) ;

return tokentype = '(';

}

} else if (c == '[') {

for

(*p++

= c;

<*P++ = getch()) I= ']'; )

;

= '\.0';

return tokentype

= BRACKETS;

} else if (isalpha(c)) {

for

(*P++

= c;

isalnum(c = getch());

*P++

= c;

*P = '\.0';

ungetch ( c ) ;

return tokentype

= NAME;

} else

= c;

return tokentype

}

getch and ungetch were discussed in Chapter 4.

Going in the other direction is easier, especially if we do not worry about

generating redundant parentheses. The program undcl converts a word

126 POINTERS ANP ARRAYS CHAPTERS

description like "x is a function returning a pointer to an array of pointers to

functions returning char," which we will express as

x () * [] * ()char

char (*(*x())[])()

The abbreviated input syntax lets us reuse the gettoken function. undcl also uses

the same external variables as del does.

I* undcl: convert word description to declaration */ main()

{

. int type;

char temp[MAXTOKEN];

while (gettoken{) I= EOF) {

strcpy(out, token);

while ((type= gettoken()) I= '\n')

if (type == PARENS I I type == BRACKETS)

strcat(out, token);

else if (type== '*') {

sprintf(temp, "(*%s)", out);

strcpy(out, temp);

} else if (type == NAME) { sprintf(temp, "%s %s", token,

out);

strcpy(out, temp); } else

printf("invalid input at %s\n", token); printf("%s\n", out);

}

return 0;

}

Exercise 5-18. Make del recover from input errors. 0

Exercise 5-19. Modify undcl so that it does not add redundant parentheses to

declarations. o

Exercise 5-20. Expand del to handle declarations with function argument types,

qualifiers like const, and so on. 0

cHAPTER a: Structures

A structure is a collection of one or more variables, possibly of different

types, grouped together under a single name for convenient handling. (Struc-

tures are called "records" in some languages, notably Pascal.) Structures help to

organize complicated data, particularly in large programs, because they per-mit a

group of related variables to be treated as a unit instead of as separate entities.

One traditional example of a structure is the payroll record: an employee is

described by a set of attributes such as name, address, social security number,

salary, etc. Some of these in turn could be structures: a name has several com-

ponents, as does an address and even a salary. Another example, more typical for

C, comes from graphics: a point is a pair of coordinates, a rectangle is a pair of

points, and so on.

The main change made by the ANSI standard is to define structure

assignment-structures may be copied and assigned to, passed to functions, and

returned by functions. This has been supported by most compilers for many

years, but the properties are now precisely defined. Automatic structures and

arrays may now also be initialized.

6. 1 Basics of Structures

Let us create a few structures suitable for graphics. The basic object is a

point, which we will assume has an x coordinate and a y coordinate, both

integers.

• (4,3)

(0,0)

127

118 STRUCTURES CHAPTER 6

The two components can be placed in a structure declared like this:

struct point {

int x;

int y;

} ;

The keyword struct introduces a structure declaration, which is a list of

declarations enclosed in braces. An optional name called a structure tag may

follow the word struct (as with point here). The tag names this kind of structure,

and can be used subsequently as a shorthand for the part of the declaration in

braces.

The variables named in a structure are called members. A structure member or

tag and an ordinary (i.e., non-member) variable can have the same name without

conflict, since they can always be distinguished by context. Furthermore, the

same member names may occur in different structures, although as a matter of

style one would normally use the same names only for closely related objects.

A struct declaration defines a type. The right brace that terminates the list of

members may be followed by a list of variables, just as for any basic type. That

is,

struct { . . . } x, y, z ;

is syntactically analogous to

int x, y, z;

in the sense that each statement declares x, y and z to be variables of the named

type and causes space to be set aside for them.

A structure declaration that is not followed by a list of variables reserves no

storage; it merely describes a template or the shape of a structure. If the

declaration is tagged, however, the tag can be used later in definitions of

instances of the structure. For example, given the declaration of point above,

struct point pt;

defines a variable pt which is a structure of type struct point. A structure can be

initialized by following its definition with a list of initializers, each a con-stant

expression, for the members:

struct point maxpt = { 320, 200 };

An automatic structure may also be initialized by assignment or by calling a

function that returns a structure of the right type.

A member of a particular structure is referred to in an expression by a con-

struction of the form

structure-name • member

The structure member operator " • " connects the structure name and the member

name. To print the coordinates of the point pt, for instance,

SECTION 6.2 STRUCTURES AND FUNCTIONS 129

printf( "%d,%d", pt.x, pt.y);

or to compute the distance from the origin (0,0) to pt,

double dist, sqrt(double);

dist = sqrt((double)pt.x * pt.x + (double)pt.y * pt.y);

Structures can be nested. One representation of a rectangle is a pair of points

that denote the diagonally opposite corners:

y Dpt2

pt1

struct rect {

struct point pt1;

struct point pt2;

} ;

The rect structure contains two point structures. If we declare screen as

struct rect screen;

then

screen.pt1.x

refers to the x coordinate of the pt 1 member of screen.

6.2 Structures and Functions

The only legal operations on a structure are copying it or assigning to it as a

unit, taking its address with &., and accessing its members. Copy and assign-

ment include passing arguments to functions and returning values from func-

tions as well. Structures may not be compared. A structure may be initialized by

a list of constant member values; an automatic structure may also be initial-ized

by an assignment.

Let us investigate structures by writing some functions to manipulate points

and rectangles. There are at least three possible approaches: pass components

separately, pass an entire structure, or pass a pointer to it. Each has its good

points and bad points.

The first function, makepoint, will take two integers and return a point

structure:

130 STRUCTURES CHAPTER 6

/* makepoint: make a point from x and y components */ struct point

makepoint(int x, int y)

{

struct point temp;

temp.x = x;

temp.y = y;

return temp;

}

Notice that there is no conflict between the argument name and the member with the

same name; indeed the re-use of the names stresses the relationship.

makepoint can now be used to initialize any structure dynamically, or to provide

structure arguments to a function:

struct rect screen;

struct point middle;

struct point makepoint(int, int);

screen.pt1 = makepoint(O, 0); screen.pt2 =

makepoint(XMAX, YMAX);

middle= makepoint((screen.pt1.x + screen.pt2.x)/2, (screen.pt1.y +

screen.pt2.y)/2);

The next step is a set of functions to do arithmetic on points. For instance,

/* addpoint: add two points */

struct point addpoint(struct point p1, struct point p2)

{

p 1 , X + = p2, X;

p1.y += p2.y;

return p1;

}

Here both the arguments and the return value are structures. We incremented the

components in p 1 rather than using an explicit temporary variable to emphasize that

structure parameters are passed by value like any others.

As another example, the function ptinrect tests whether a point is inside a

rectangle, where we have adopted the convention that a rectangle includes its left

and bottom sides but not its top and right sides:

/* ptinrect: return 1 if int

ptinrect(struct point

{

return p.x >= r.pt1.x && p.y

>= r.pt1.y

}

p in r, 0 if not */ p, struct

rect r)

&& p.x < r.pt2.x

&& p.y < r.pt2.y;

This assumes that the rectangle is represented in a standard form where the pt 1

coordinates are less than the pt2 coordinates. The following function. returns a

rectangle guaranteed to be in canonical form:

SECTION 6.2

STRUCTURES AND FUNCTIONS

131

#define min(a,

#define max(a,

((a)

(b)

? (a)

(b))

/* canonrect: canonicalize coordinates of rectangle */ struct rect canonrect(struct rect r)

{

struct rect temp;

temp.pt1.x = min(r.pt1.x, r.pt2.x); temp.pt1.y =

min(r.pt1.y, r.pt2.y); temp.pt2.x = max(r.pt1.x,

r.pt2.x); temp.pt2.y = max(r.pt1.y, r.pt2.y); return

temp;

}

If a large structure is to be passed to a function, it is generally more efficient to

pass a pointer than to copy the whole structure. Structure pointers are just like

pointers to ordinary variables. The declaration

struct point *pp;

says that pp is a pointer to a structure of type struct point. If pp points to

a point structure, *PP is the structure, and ( *PP) . x and ( *PP) • y are the members.

To use pp, we might write, for example,

struct point origin, *PP;

pp = &.origin;

printf("origin is (%d,%d)\n", (*pp).x, (*pp).y);

The parentheses are necessary in ( *PP) • x because the precedence of the struc-ture

member operator . is higher than *. The expression *PP. x means

* ( pp. x) , which is illegal here because x is not a pointer.

Pointers to structures are so frequently used that an alternative notation is

provided as a shorthand. If p is a pointer to a structure, then

p->member-of-structure

refers to the particular member. (The operator - > is a minus sign immediately

followed by > .) So we could write instead

printf("origin is (%d,%d)\n", pp->x, pp->y);

Both • and -> associate from left to right, so if we have

struct rect r, *rp = &.r;

then these four expressions are equivalent:

r .pt 1.x

rp->pt 1. x (

r. pt 1). x

( rp->pt 1) .x

131 STRUCTURES CHAPTER 6

The structure operators • and ->, together with ( ) for function calls and [] for

subscripts, are at the top of the precedence hierarchy and thus bind very tightly.

For example, given the declaration

struct {

int len;

char •str;

} •p;

then

++p->len

increments len, not p, because the implied parenthesization is ++(p->len).

Parentheses can be used to alter the binding: ( ++p) ->len increments p before

accessing 1 en, and ( p+ + ) - >1 en increments p afterward. (This last set of

parentheses is unnecessary.)

In the same way, •p->str fetches whatever str points to; •p->str++ increments

str after accessing whatever it points to (just like •s++); ( •p->str) ++increments

whatever str points to; and *P++->str increments p after accessing whatever str

points to.

6.3 Arrays of Structures

Consider writing a program to count the occurrences of each C keyword. We

need an array of character strings to hold the names, and an array of integers for

the counts. One possibility is to use two parallel arrays, keyword and keycount,

as in

char •keyword[NKEYS];

int keycount[NKEYS];

But the very fact that the arrays are parallel suggests a different organization, an

array of structures. Each keyword entry is a pair:

char •word;

int count;

and there is an array of pairs. The structure declaration

struct key {

char •word;

int count;

} keytab[NKBYS];

declares a structure type key, defines an array keytab of structures of this type,

and sets aside storage for them. Each element of the array is a structure. This

could also be written

SECTION 6.3 ARRAYS OF STRUCTURES 133

struct key {

char *WOrd;

int count;

} ;

struct key keytab[NKEYS];

Since the structure keytab contains a constant set of names, it is easiest to

make it an external variable and initialize it once and for all when it is defined.

The structure initialization is analogous to earlier ones-the definition is fol-lowed

by a list of initializers enclosed in braces:

struct key {

char *WOrd;

int count;

} keytab[] = { "auto",

.o, "break", 0,

"case", 0, "char",

0, "const", 0,

"continue", 0,

"default", 0,

/* •.. */

"unsigned", 0,

"void", 0,

"volatile", 0,

"while", 0

} ;

The initializers are listed in pairs corresponding to the structure members. It

would be more precise to enclose initializers for each "row" or structure in braces,

as in

{ "auto", 0 }, {

"break", 0 }, {

"case", 0 },

but the inner braces are not necessary when the initializers are simple variables

or character strings, and when all are present. As usual, the number of entries in

the array keytab will be computed if initializers are present and the [ ] is left

empty.

The keyword-counting program begins with the definition of keytab. The

main routine reads the input by repeatedly calling a function getword that fetches

one word at a time. Each word is looked up in keytab with a version of the binary

search function that we wrote in Chapter 3. The list of keywords must be sorted

in increasing order in the table.

134 STRUCTURES CHAPTER6

#include <stdio.h>

#include <ctype.h>

#include <string.h>

#define MAXWORD 100

int getword(char *' int);

int binsearch(char *, struct key*' int);

I* count C keywords */

main()

{

int n;

char word[MAXWORD];

while (getword(word, MAXWORD) I= EOF)

if (isalpha(word[O]))

if ((n = binsearch(word, keytab, NKEYS)) >= 0)

keytab[n].count++;

for (n = 0; n < NKEYS; n++)

if (keytab[n].count > 0)

printf( "%4d %s\n",

keytab[n].count, keytab[n].word);

return 0;

}

I* binsearch: find word in tab[O] •.. tab[n-1] */ int binsearch(char

*WOrd, struct key tab[], int n)

{

int cond;

int low, high, mid;

low = 0;

high = n - 1;

while (low <= high) {

mid = (low+high) I 2;

if ((cond = strcmp(word, tab[mid].word)) < 0)

high = mid - 1 ;

else if (cond > 0)

low = mid + 1;

else

return mid;

}

return -1;

}

We will show the function qetword in a moment; for now it suffices to say that each

call to qetword finds a word, which is copied into the array named as its first

argument.

The quantity NKEYS is the number of keywords in keytab. Although we

SECTION 6.3 ARRAYS OF STRUCTURES 135

could count this by hand, it's a lot easier and safer to do it by machine, espe-

cially if the list is subject to change. One possibility would be to terminate the

list of initializers with a null pointer, then loop along keytab until the end is

found.

But this is more than is needed, since the size of the array is completely

determined at compile time. The size of the array is the size of one entry times

the number of entries, so the number of entries is just

size of keytab I size of struct key

C provides a compile-time unary operator called sizeof that can be used to

compute the size of any object. The expressions

sizeof object

and

sizeof (type name)

yield an integer equal to the size of the specified object or type in bytes. (Strictly,

sizeof produces an unsigned integer value whose type, size_ t, is defined in the

header <stddef. h>.) An object can be a variable or array or structure. A type

name can be the name of a basic type like int or double, or a derived type like a

structure or a pointer.

In our case, the number of keywords is the size of the array divided by the

size of one element. This computation is used in a #define statement to set the

value of NKEYS:

#define NKEYS (sizeof keytab I sizeof(struct key))

Another way to write this is to divide the array size by the size of a specific ele-

ment:

#define NKEYS (sizeof keytab I sizeof keytab[O])

This has the advantage that it does not need to be changed if the type changes. A

sizeof can not be used in a #if line, because the preprocessor does not

parse type names. But the expression in the #define is not evaluated by the

preprocessor, so the code here is legal.

Now for the function getword. We have written a more general getword than is

necessary for this program, but it is not complicated. getword fetches the next

"word" from the input, where a word is either a string of letters and digits

beginning with a letter, or a single non-white space character. The func-tion

value is the first character of the word, or EOF for end of file, or the char-acter

itself if it is not alphabetic.

136 STRUCTURES CHAPTER6

/* qetword: get next word or character from input */ int qetword(char

*WOrd, int lim)

{

int c, qetch(void);

void unqetch(int);

char *W = word;

while (isspace(c = qetch()))

if (c I= EOF)

*W++ = c;

if (lisalpha(c)) {

*W = '\0';

return c;

}

for ( ; --lim > 0; w++)

if ( lisalnum(*w = getch())) {

unqetch ( *W) ;

break;

}

*W = '\0';

return word[O];

}

getword uses the getch and ungetch that we wrote in Chapter 4. When the collection

of an alphanumeric token stops, getword has gone one character too far. The call to

ungetch pushes that character back on the input for the next call. getword also uses

isspace to skip white space, isalpha to iden-tify letters, and isalnum to identify letters

and digits; all are from the stand-ard header <ctype. h>.

Exercise 6-l. Our version of getword does not properly handle underscores, string

constants, comments, or preprocessor control lines. Write a better ver-sion. D

6.4 Pointers to Structures

To illustrate some of the considerations involved with pointers to and arrays

of structures, let us write the keyword-counting program again, this time using

pointers instead of array indices.

The external declaration of keytab need not change, but main and binsearch do

need modification.

SECTION 6.4 POINTERS TO STRUCTURES 137

#include <stdio.h>

#include <ctype.h>

#include <string.h>

#define MAXWORD 100

int getword(char *• int);

struct key *binsearch(char *• struct key *• int);

/* count C keywords; pointer version */ main()

{

char word[MAXWORD];

struct key *Pi

while (getword(word, MAXWORD) I= EOF)

if (isalpha(word[O]))

if ((p=binsearch(word, keytab, NKEYS)) I= NULL)

p->count++;

for (p = keytab; p < keytab + NKEYS; p++)

if (p->count > 0)

printf( "%4d %s\n", p->count, p->word);

return 0;

}

/* binsearch: find word in tab[O] .•. tab[n-1] */

struct key *binsearch(char *WOrd, struct key *tab, int n)

{

int cond;

struct key *low= &tab[O];

struct key *high= &tab[n];

struct key *mid;

while (low < high) {

mid = low + (high-low) I 2;

if ((cond = strcmp(word, mid->word)) < 0)

high = mid;

else if (cond > 0)

low = mid + 1;

else

return mid;

}

return NULL;

}

There are several things worthy of note here. First, the declaration of

binsearch must indicate that it returns a pointer to struct key instead of an integer;

this is declared both in the function prototype and in binsearch. If binsearch finds the

word, it returns a pointer to it; if it fails, it returns

NULL.

Second, the elements of keytab are now accessed by pointers. This

138 STRUCTURES CHAPTER 6

requires significant changes in binsearch.

The initializers for low and high are now pointers to the beginning and just

past the end of the table.

The computation of the middle element can no longer be simply

mid = (low+high) I 2

because the addition of two pointers is illegal. Subtraction is legal, however, so

high-low is the number of elements, and thus

mid = low + (high-low) I 2

sets mid to point to the element halfway between low and high.

The most important change is to adjust the algorithm to make sure that it does

not generate an illegal pointer or attempt to access an element outside the array.

The problem is that &.tab[ -1] and &.tab[n] are both outside the lim-its of the array

tab. The former is strictly illegal, and it is illegal to derefer-ence the latter. The

language definition does guarantee, however, that pointer arithmetic that

involves the first element beyond the end of an array (that is, &.tab[n]) will work

correctly.

In main we wrote

for (p = keytab; p < keytab + NKEYS; p++)

If p is a pointer to a structure, arithmetic on p takes into account the size of the

structure, so p++ increments p by the correct amount to get the next element of

the array of structures, and the test stops the loop at the right time.

Don't assume, however, that the size of a structure is the sum of the sizes of

its members. Because of alignment requirements for different objects, there may

be unnamed "holes" in a structure. Thus, for instance, if a char is one byte and an

int four bytes, the structure

struct {

char c;

int i;

} ;

might well require eight bytes, not five. The sizeof operator returns the proper

value.

Finally, an aside on program format: when a function returns a complicated

type like a structure pointer, as in

struct key *binsearch(char *Word, struct key *tab, int n)

the function name can be hard to see, and to find with a text editor. Accord-ingly

an alternate style is sometimes used:

struct key *

binsearch(char *WOrd, struct key *tab, int n)

This is a matter of personal taste; pick the form you like and hold to it.

SECTION 6.5 SELF-REFERENTIAL STRUCTURES 139

6.5 Self-referential Structures

Suppose we want to handle the more general problem of counting the

occurrences of all the words in some input. Since the list of words isn't known in

advance, we can't conveniently sort it and use a binary search. Yet we can't do a

linear search for each word as it arrives, to see if it's already been seen; the

program would take too long. (More precisely, its running time is likely to grow

quadratically with the number of input words.) How can we organize the data to

cope efficiently with a list of arbitrary words?

One solution is to keep the set of words seen so far sorted at all times, by

placing each word into its proper position in the order as it arrives. This shouldn't

be done by shifting words in a linear array, though-that also takes too long.

Instead we will use a data structure called a binary tree.

The tree contains one "node" per distinct word; each node contains

a pointer to the text of the word

a count of the number of occurrences

a pointer to the left child node

a pointer to the right child node

No node may have more than two children; it might have only zero or one.

The nodes are maintained so that at any node the left subtree contains only

words that are lexicographically less than the word at the node, and the right

subtree contains only words that are greater. This is the tree for the sentence

"now is the time for all good men to come to the aid of their party", as built by

inserting each word as it is encountered:

now

/""-

the

fo/ ~n o( \ime

/ \ \ I\

all

good

party their to

aid come

To find out whether a new word is already in the tree, start at the root and

compare the new word to the word stored at that node. If they match, the ques-

tion is answered affirmatively. If the new word is less than the tree word, con-

tinue searching at the left child, otherwise at the right child. If there is no child in

the required direction, the new word is not in the tree, and in fact the empty slot

is the proper place to add the new word. This process is recursive, since the

search from any node uses a search from one of its children. Accordingly,

recursive routines for insertion and printing will be most natural.

Going back to the description of a node, it is conveniently represented as a

structure with four components:

140 STRUCTURES

CHAPTER 6

struct tnode {

/* the

tree

node: */

char

*word;

points to the text */

int

count;

/* number of occurrences */

struct

tnode

*left;

left

child */

struct

tnode

*right;

right

child */

} ;

This recursive declaration of a node might look chancy, but it's correct. It is

illegal for a structure to contain an instance of itseif, but

struct tnode *left;

declares left to be a pointer to a tnode, not a tnode itself.

Occasionally, one needs a variation of self-referential structures: two struc-

tures that refer to each other. The way to handle this is:

struct t {

struct s *P; /* p points to an s */

} ;

struct s {

struct t *q; /* q points to a t */

} ;

The code for the whole program is surprisingly small, given a handful of sup-

porting routines like getword that we have already written. The main routine

reads words with getword and installs them in the tree with addtree.

#include <stdio.h>

#include <ctype.h>

#include <string.h>

#define MAXWORD 100

struct tnode *addtree(struct tnode *• char*); void

treeprint(struct tnode *);

int getword(char *• int);

/* word frequency count */

main()

{

struct tnode *root;

char word[MAXWORD];

root • NULL;

while (getword(word, MAXWORD) I= EOF)

if (isalpha(word[O]))

root= addtree(root, word);

treeprint(root);

return 0;

}

SECTION 6.5 SELF-REFERENTIAL STRUCTURES 141

The function addtree is recursive. A word is presented by main to the top level

(the root) of the tree. At each stage, that word is compared to the word already

stored at the node, and is percolated down to either the left or right sub-tree by a

recursive call to addtree. Eventually the word either matches some-thing already

in the tree (in which case the count is incremented), or a null pointer is

encountered, indicating that a node must be created and added to the tree. If a

new node is created,- addtree returns a pointer to it, which is installed in the

parent node.

struct tnode *talloc(void);

char *Strdup(char *);

/* addtree: add a node with w, at or below p */ struct tnode

*addtree(struct tnode *P• char *W)

{

int

cond;

NULL) {

/* a new word has arrived */

= talloc();

/*make a new node*/

p->word = strdup(w);

p->count = 1;

p->left = p->right = NULL;

} else if ((cond

= strcmp(w, p->word)) == 0)

p->count++;

/* repeated word */

else

if (cond <

/* less than into left subtree */

p->left

= addtree(p->left, w);

else

greater than into right subtree */

p->right

= addtree(p->right, w);

return p;

}

Storage for the new node is fetched by a routine talloc, which returns a

pointer to a free space suitable for holding a tree node, and the new word is

copied to a hidden place by strdup. (We will discuss these routines in a moment.)

The count is initialized, and the two children are made null. This part of the code

is executed only at the leaves of the tree, when a new node is being added. We

have (unwisely) omitted error checking on the values returned by strdup and

talloc.

treeprint prints the tree in sorted order; at each node, it prints the left subtree (all

the words less than this word), then the word itself, then the right subtree (all the

words greater). If you feel shaky about how recursion works, simulate treeprint as it

operates on the tree shown above.

142 STRUCTURES CHAPTER 6

/* treeprint: in-order print of tree p */ void treeprint(struct

tnode *P)

{

if (p I= NULL) {

treeprint(p->left);

print£( "%4d %s\n", p->count, p->word); treeprint(p-

>right);

}

A practical note: if the tree becomes "unbalanced" because the words don't

arrive in random order, the running time of the program can grow too much. As a

worst case, if the words are already in order, this program does an expen-sive

simulation of linear search. There are generalizations of the binary tree that do

not suffer from this worst-case behavior, but we will not describe them here.

Before we leave this example, it is also worth a brief digression on a problem

related to storage aUocators. Clearly it's de~irable that there be only one storage

allocator in a program, even though it 'allocates different kinds of objects. But if

one alloeator is to process req'!lests for, say, pointers to chars and pointers to

struct tnodes, two questioqs !lrise. First, how does it meet the requirement of

most real machines that objects of certain types must satisfy alignment·

restrictions (for example, integers often must be located at even addresses)?

Secong, what declarations can cope with the fact that an allocator must

necessarily return different kinds of pointers~

Alignment requirements can generally be ~atisfied easily, at the cost of some

wasted space, by ensuring that the allocator always returns a pointer that meets

all alignment restrictions. The alloc of Chapter 5 does not guarantee any particular

alignment, so we will use the standard library function malloc, which does. In

Chapter 8 we will show OIJ.e way to implement malloc.

The question of the type declaration for a function like malloc is a vexing one

for any language that takes its type-checking seriously. In C, the proper method

is to declare that malloc ret~rns a pojnter to void, then explicitly coerce the pointer

into the desired type with a cast. malloc and related rou-tines are declared in the

standard header <stdlib. h>. Thus talloc can be written as

#include <$tdlib.h>

/* talloc: make ~ tnode */

struct tnod~ *talloc(void)

{

return (struct tnode *> mallop(sizeof(struct tnode));

}

strdup merely copies the string given by its argument into a safe place,

obtained by a call on m~lloc:

SECTION 6.6 TABLE LOOKUP 143

char *Strdup(char *S)

make a duplicate of s */

{

char *P;

p = (char*) rnalloc(strlen(s)+1);

/* +1 for '\0'

if (p I= NULL)

strcpy ( p, s ) ;

return p;

}

malloc returns NULL if no space is available; strdup passes that value on, leaving

error-handling to its caller.

Storage obtained by calling malloc may be freed for re-use by calling free; see

Chapters 7 and 8.

Exercise 6-2. Write a program that reads a C program and prints in alphabeti-cal

order each group of variable names that are identical in the first 6 charac-ters, but

different somewhere thereafter. Don't count words within strings and comments.

Make 6 a parameter that can be set from the command line. D

Exercise 6-3. Write a cross-referencer that prints a list of all words in a docu-

ment, and, for each word, a list of the line numbers on which it occurs. Remove

noise words like "the," "and," and so on. D

Exercise 6-4. Write a program that prints the distinct words in its input sorted

into decreasing order of frequency of occurrence. Precede each word by its

count. D

6. 6 Table Lookup

In this section we will write the innards of a table-lookup package, to illus-

trate more aspects of structures. This code is typical of what might be found in

the symbol table management routines of a macro processor or a compiler. For

example, consider the #define statement. When a line like

#define IN 1

is encountered, the name IN and the replacement text 1 are stored in a table.

Later, when the name IN appears in a statement like

state = IN;

it must be replaced by 1.

There are two routines that manipulate the names and replacement texts.

install ( s, t) records the name s and the replacement text t in a table; s and t are

just character strings. lookup( s) searches for s in the table, and returns a pointer

to the place where it was found, or NULL if it wasn't there.

The algorithm is a hash search-the incoming name is converted into a small

144 STRUCTURES CHAPTER 6

non-negative integer, which is then used to index into an array of pointers. An

array element points to the beginning of a linked list of blocks describing names

that have that hash value. It is NULL if no names have hashed to that value.

name

defn

A block in the list is a structure containing pointers to the name, the

replacement text, and the next block in the list. A null next-pointer marks the end

of the list.

struct nlist {

table

entry: */

struct nlist

*next;

next entry in chain */

char

*name;

defined name

char

*defn;

replacement

text */

} ;

The pointer array is just

#define HASHSIZE 101

static struct nlist *hashtab[HASHSIZE]; /*pointer table*/

The hashing function, which is used by both lookup and install, adds each

character value in the string to a scrambled combination of the previous ones and

returns the remainder modulo the array size. This is not the best pos-sible hash

function, but it is short and effective.

I* hash: form hash value for string s */ unsigned hash(char

*S)

{

unsigned hashval;

for (hashval = 0; *S I= '\0'; s++)

hashval = *S + 31 * hashval;

return hashval % HASHSIZE;

}

Unsigned arithmetic ensures that the hash value is non-negative.

The hashing process produces a starting index in the array hashtab; if the

string is to be found anywhere, it will be in the list of blocks beginning there. The

search is performed by lookup. If lookup finds the entry already present, it returns

a pointer to it; if not, it returns NULL.

SECTION 6.6 TABLE LOOKUP 145

/* lookup: look for s in hashtab */ struct nlist

*lookup(char *S)

{

struct nlist *np;

for (np = hashtab[hash(s)]; np I= NULL; np = np->next)

if (strcmp(s, np->name) == 0)

return np; /* found */

return NULL; /* not found */

}

The for loop in lookup is the standard idiom for walking along a linked list:

for (ptr = head; ptr I= NULL; ptr = ptr->next)

install uses lookup to determine whether the name being installed is already

present; if so, the new definition will supersede the old one. Otherwise, a new entry

is created. install returns NULL if for any reason there is no room for a new entry.

struct nlist *lookup(char *);

char *Strdup(char *);

/* install: put (name, defn) in hashtab */ struct nlist

*install(char *name, char *defn)

{

struct nlist *np;

unsigned hashval;

if ((np = lookup(name)) ==NULL) { /*not found*/

np = (struct nlist *) malloc(sizeof(*np));

if (np ==NULL :: (np->name = strdup(name)) ==NULL)

return NULL;

hashval = hash(name);

np->next = hashtab[hashval];

hashtab[hashval] = np;

} else /* already there */

free((void *) np->defn); /*free previous defn */

if ((np->defn = strdup(defn)) ==NULL)

return NULL;

return np;

}

Exercise 6-5. Write a function undef that will remove a name and definition from

the table maintained by lookup and install. o

Exercise 6-6. Implement a simple version of the #define processor (i.e., no

arguments) suitable for use with C programs, based on the routines of this sec-

tion. You may also find getch and ungetch helpful. 0

146 STRUCTURES CHAPTER 6

6.7 Typedef

C provides a facility called typedef for creating new data type names. For

example, the declaration

typedef int Length;

makes the name Length a synonym for int. The type Length can be used in

declarations, casts, etc., in exactly the same ways that the type int can be:

Length len, maxlen;

Length *lengths[];

Similarly, the declaration

typedef char *String;

makes String a synonym for char *or character pointer, which may then be used in

declarations and casts:

String p, lineptr[MAXLINES], alloc(int); int·

strcmp(String, String); p = (String) malloc(100);

Notice that the type being declared in a typedef appears in the position of a

variable name, not right after the word typedef. Syntactically, typedef is like the

storage classes extern, static, etc. We have used capitalized names for typedefs, to

make them stand out.

As a more complicated example, we could make typedefs for the tree

nodes shown earlier in this chapter:

typedef

struct

tnode

*Treeptr;

typedef

struct

tnode

{

/* the

tree

node: *I

char

*WOrd;

/* points to the text */

int

count;

/* number of occurrences *I

Treeptr left;

left

child */

Treeptr right;

right

child *I

} Treenode;

This creates two new type keywords called Treenode (a structure) and Treeptr (a

pointer to the structure). Then the routine talloc could become

Treeptr talloc(void)

{

return (Treeptr) malloc(sizeof(Treenode));

}

It must be emphasized that a typedef declaration does not create a new type in

any sense; it merely adds a new name for some existing type. Nor are there any new

semantics: variables declared this way have exactly the same pro-perties as variables

whose declarations are spelled out explicitly. In effect, typedef is like #define,

except that since it is interpreted by the compiler, it

SECTION 6.8 UNIONS 147

can cope with textual substitutions that are beyond the capabilities of the

preprocessor. For example,

typedef int (*PFI)(char *• char*);

creates the type PFI, for "pointer to function (of two char * arguments) returning

int," which can be used in contexts like

PFI strcmp, numcmp;

in the sort program of Chapter 5.

Besides purely aesthetic issues, there are two main reasons for using typedefs.

The first is to parameterize a program against portability problems. If typedefs are

used for data types that may be machine-dependent, only the typedefs need change

when the program is moved. One common situation is to use typedef names for various

integer quantities, then make an appropriate set of choices of short, int, and long for

each host machine. Types like siz~_1;: and ptrdiff_t from the standard library are

examples.

The second purpose of typedefs is to provide better documentation for a

program-a type called Treeptr may be easier to understand than one declared only as

a pointer to a complicated structure.

6.8 Unions

A union is a variable that may hold (at different times) objects of different types

and sizes, with the compiler keeping track of size and alignment require-ments.

Unions provide a way to manipulate different kinds of data in a single area of

storage, without embedding any machine-dependent information in the program.

They are analogous to variant records in Pascal.

As an example such as might be found in a compiler symbol table manager,

suppose that a constant may be an int, a float, or a character pointer. The value of a

particular constant must be stored in a variable of the proper type, yet it is most

convenient for table management if the value occupies the same amount of storage

and is stored in the same place regardless of its type. This is the purpose of a union-a

single variable that can legitimately hold any one of several types. The syntax is

based on structures:

union u_tag {

int ival;

float fval;

char *SVal;

} u;

The variable u will be large enough to hold the largest of the three types; the

specific size is implementation-dependent. Any one of these types may be assigned

to u and then used in expressions, so long as the usage is consistent: the type

retrieved must be the type most recently stored. It is the programmer's

148 STRUCTURES CHAPTER6

responsibility to keep track of which type is currently stored in a union; the

results are implementation-dependent if something is stored as one type and

extracted as another.

Syntactically, members of a union are accessed as

union-name • member

union-pointer-> member

just as for structures. If the variable utype is used to keep track of the current type

stored in u, then one might see code such as

if (utype == INT)

printf ( "%d\n", u. ival);

else if (utype == FLOAT)

printf ( "%f\n", u. fval) ;

else if (utype == STRING)

printf( "%s\n", u. sval);

else

printf( "bad type %d in utype\n", utype);

Unions may occur within structures and arrays, and vice versa. The notation

for accessing a member of a union in a structure (or vice versa) is identical to

that for nested structures. For example, in the structure array defined by

struct {

char *name;

int flags;

int utype;

union {

int ival;

float fval;

char *Sval;

} u;

} symtab[NSYM];

the member i val is referred to as

symtab[i].u.ival

and the first character of the string sval by either of

*Symtab[i].u.sval

symtab[i].u.sval[O]

In effect, a union is a structure in which all members have offset zero from

the base, the structure is big enough to hold the "widest" member, and the

alignment is appropriate for all of the types in the union .. The same operations

are permitted on unions as on structures: assignment to or copying as a unit,

taking the address, and accessing a member.

A union may only be initialized with a value of the type of its first member;

SECTION 6.9 BIT-FIELDS 149

thus the union u described above can only be initialized with an integer value.

The storage allocator in Chapter 8 shows how a union can be used to force a

variable to be aligned on a particular kind of storage boundary.

6.9 Bit-fields

When storage space is at a premium, it may be necessary to pack several

objects into a single machine word; one common use is a set of single-bit flags

in applications like compiler symbol tables. Externally-imposed data formats,

such as interfaces to hardware devices, also often require the ability to get at

pieces of a word.

Imagine a fragment of a compiler that manipulates a symbol table. Each

identifier in a program has certain information associated with it, for example,

whether or not it is a keyword, whether or not it is external and/or static, and so

on. The most compact way to encode such information is a set of one-bit flags in

a single char or int.

The usual way this is done is to define a set of "masks" corresponding to the

relevant bit positions, as in

#define KEYWORD 01

#define EXTERNAL 02

#define STATIC 04

enum { KEYWORD = 01, EXTERNAL = 02, STATIC

= 04 };

The numbers must be powers of two. Then accessing the bits becomes a matter

of "bit-fiddling" with the shifting, masking, and complementing operators that

were described in Chapter 2.

Certain idioms appear frequently:

flags :: EXTERNAL : STATIC;

turns on the EXTERNAL and STATIC bits in flags, while

flags&= -(EXTERNAL : STATIC);

turns them off, and

if ( (flags & (EXTERNAL : STATIC) ) = = 0 ) ...

is true if both bits are off.

Although these idioms are readily mastered, as an alternative C offers the

capability of defining and accessing fields within a word directly rather than by

bitwise logical operators. A bit-field, or field for short, is a set of adjacent bits

within a single implementation-defined storage unit that we will call a "word."

The syntax of field definition and access is based on structures. For example, the

symbol table #defines above could be replaced by the definition of three

ISO STRUCTURES CHAPTER 6

fields:

struct {

unsigned int is_keyword

unsigned int is_extern unsigned

int is static

} flags;

1 ;

1 •

•

This defines a variable called flags that contains three 1-bit fields. The number

following the colon represents the field width in bits. The fields are declared

unsigned int to ensure that they are unsigned quantities.

Individual fields are referenced in the same way as other structure members:

flags. is_keyword, flags. is_extern, etc. fields behave like small integers, and may

participate in arithmetic expressions just like other integers. Thus the previous

examples may be written more naturally as

flags.is_extern = flags.is_static = 1;

to turn the bits on;

flags.is_extern = flags.is_static = 0;

to turn them off; and

if (flags.is_extern -- 0 && flags.is_static -- 0)

to test them.

Almost everything about fields is implementation-dependent. Whether a field

may overlap a word boundary is implementation-defined. Fields need not be named;

unnamed fields (a colon and width only) are used for padding. The special width 0

may be used to force alignment at the next word boundary.

Fields are assigned left to right on some machines and right to left on others.

This means that although fields are useful for maintaining internally-defined data

structures, the question of which end comes first has to be carefully con-sidered

when picking apart externally-defined data; programs that depend on such things are

not portable. Fields may be declared only as ints; for portabil-ity, specify signed or

unsigned explicitly. They are not arrays, and they do not have addresses, so the &

operator cannot be applied to them.

cHAPTER 1: Input and Output

Input and output facilities are not part of the C language itself, so we have

not emphasized them in our presentation thus far. Nonetheless, programs interact

with their environment in much more complicated ways than those we have

shown before. In this chapter we will describe the standard library, a set of

functions that provide input and output, string handling, storage manage-ment,

mathematical routines, and a variety of other services for C programs. We will

concentrate on input and output.

The ANSI standard defines these library functions precisely, so that they can

exist in compatible form on any system where C exists. Programs that confine

their system interactions to facilities provided by the standard library can be

moved from one system to another without change.

The properties of library functions are specified in more than a dozen

headers; we have already seen several of these, including <stdio. h>, <string. h>,

and <ctype. h>. We will not present the entire library here, since we are more

interested in writing C programs that use it. The library is described in detail in

Appendix B.

7. 1 Standard Input and Output

As we said in Chapter 1, the library implements a simple model of text input

and output. A text stream consists of a sequence of lines; each line ends with a

newline character. If the system doesn't operate that way, the library does

whatever is necessary to make it appear as if it does. For instance, the library

might convert carriage return and linefeed to newline on input and back again on

output.

The simplest input mechanism is to read one character at a time from the

standard input, normally the keyboard, with getchar:

int getchar(void)

getchar returns the next input character each time it is called, or EOF when it

encounters end of file. The symbolic constant EOF is defined in <stdio. h>.

151

152 INPUT AND OUTPUT CHAPTER 7

The value is typically -1, but tests should be written in terms of EOF so as to be

independent of the specific value.

In many environments, a file may be substituted for the keyboard by using

the < convention for input redirection: if a program prog uses getchar, then the

command line

prog <infile

causes prog to read characters from infile instead. The switching of the input is

done in such a way that prog itself is oblivious to the change; in partic-ular, the

string "<infile" is not included in the command-line arguments in argv. Input

switching is also invisible if the input comes from another program via a pipe

mechanism: on some systems, the command line

otherprog : prog

runs the two programs otherprog and prog, and pipes the standard output of

otherprog into the standard input for prog.

The function

int putchar(int)

is used for output: putchar (c) puts the character c on the standard output, which is

by default the screen. putchar returns the character written, or EOF if an error

occurs. Again, output can usually be directed to a file with

>filename: if prog uses putchar,

prog >outfile

will write the standard output to outfile instead. If pipes are supported,

prog : anotherprog

puts the standard output of prog into the standard input of anotherprog.

Output produced by printf also finds its way to the standard output.

Calls to putchar and printf may be interleaved-output appears in the order in

which the calls were made.

Each source file that refers to an input/output library function must contain

the line

#include <stdio.h>

before the first reference. When the name is bracketed by < and > a search is

made for the header in a standard set of places (for example, on UNIX systems,

typically in the directory /usr/include).

Many programs read only one input stream and write only one output stream;

for such programs, input and output with getchar, putchar, and printf may be

entirely adequate, and is certainly enough to get started. This is particularly true if

redirection is used to connect the output of one program to the input of the next. For

example, consider the program lower, which con-verts its input to lower case:

SECTION 7.2 FORMATTED OUTPUT-PRINTF 153

#include <stdio.h>

#include <ctype.h>

main() /*lower: convert input to lower case*/

{

int c;

while ((c = getchar()) I= EOF)

putchar(tolower(c));

return 0;

}

The function tolower is defined in <ctype. h>; it converts an upper case letter

to lower case, and returns other characters untouched. As we mentioned earlier,

"functions" like getchar and putchar in <stdio. h> and tolower in <ctype. h> are

often macros, thus avoiding the overhead of a function call per character. We

will show how this is done in Section 8.5. Regardless of how the <ctype . h>

functions are implemented on a given machine, programs that use them are

shielded from knowledge of the character set.

Exercise 7-1. Write a program that converts upper case to lower or lower case to

upper, depending on the name it is invoked with, as found in argv[ 0 ]. D

7. 2 Formatted Output-Printf

The output function printf translates internal values to characters. We have

used printf informally in previous chapters. The description here covers most

typical uses but is not complete; for the full story, see Appendix B.

int print£ (char *format, arg

, arg

, ... )

printf converts, formats, and prints its arguments on the standard output under

control of the format. It returns the number of characters printed.

The format string contains two types of objects: ordinary characters, which

are copied to the output stream, and conversion specifications, each of which

causes conversion and printing of the next successive argument to printf. Each

conversion specification begins with a % and ends with a conversion char-acter.

Between the %and the conversion character there may be, in order:

• A minus sign, which specifies left adjustment of the converted argument.

• A number that specifies the minimum field width. The converted argument will be

printed in a field at least this wide. If necessary it will be padded on the left (or right, if

left adjustment is called for) to make up the field width.

• A period, which separates the field width from the precision.

• A number, the precision, that specifies the maximum number of characters to be printed

from a string, or the number of digits after the decimal point of a floating-point value, or

the minimum number of digits for an integer.

154

INPUT AND OUTPUT

CHAPTER 7

• An

if the integer is to be printed as a

short, or 1 {letter ell) if as a

long.

Conversion characters are shown in Table 7-1. If the character after the % is not a

conversion specification, the behavior is undefined.

TABLE 7-1. BASIC PRINTF CONVERSIONS

CHARACTER

ARGUMENT TYPE;

PRINTED AS

d, i

int; decimal number.

int; unsigned octal number (without a leading zero).

x, X

int; unsigned hexadecimal number (without a leading Ox or

OX), using abcdef or ABCDEF for

10, ... , 15.

u int; unsigned decimal number.

cint; single character.

schar *; print characters from the string until a '\0' or the

number of characters given by the precision.

f double; [- ]m.dddddd, where the number of d's is given by the

precision (default 6).

e, E double; [-]m.dddddde±xx or [-]m.ddddddE±xx, where the number

of d's is given by the precision (default 6).

g, G double; use %e or %E if the exponent is less than -4 or greater than

or equal to the precision; otherwise use %£. Trailing zeros and a

trailing decimal point are not printed.

p void *; pointer (implementation-dependent representation).

%no argument is converted; print a %.

A width or precision may be specified as *• in which case the value is com-

puted by converting the next argument (which must be an int). For example, to

print at most max characters from a string s,

printf("%.*s", max, s);

Most of the format conversions have been illustrated in earlier chapters. One

exception is precision as it relates to strings. The following table shows the effect

of a variety of specifications in printing "hello, world" (12 characters). We have

put colons around each field so you can see its extent.

:%s:

:hello,

world:

:%10s:

:hello,

world:

:%.10s:

:hello,

wor:

:%-10s:

:hello,

world:

:%. 15s:

:hello,

world:

:%-15s:

:hello,

world

:%15.10s:

hello, wor:

:%-15.10s:

:hello,

wor

A warning: print£ uses its first argument to decide how many arguments

SECTION 7.3 VARIABLE-LENGTH ARGUMENT LISTS 155

follow and what their types are. It will get confused, and you will get wrong

answers, if there are not enough arguments or if they are the wrong type. You

should also be aware of the difference between these two calls:

printf(s); printf("%s",

s);

/*FAILS if s contains%*/ /*SAFE*/

The function sprint£ does the same conversions as print£ does, but stores the

output in a string:

int sprintf(char *String, char *format, arg

, arg

, ••• )

sprint£ formats the arguments in arg

, arg

, etc., according to format as before, but

places the result in string instead of on the standard output; string must be big

enough to receive the result.

Exercise 7-2. Write a program that will print arbitrary input in a sensible way.

As a minimum, it should print non-graphic characters in octal or hexadecimal

according to local custom, and break long text lines. D

7. 3 Variable-length Argument Lists

This section contains an implementation of a minimal version of print£, to

show how to write a function that processes a variable-length argument list in a

portable way. Since we are mainly interested in the argument processing,

minprintf will process the format string and arguments but will call the real print£ to

do the format conversions.

The proper declaration for print£ is

int printf(char *fmt, ... )

where the declaration .•. means that the number and types of these arguments

may vary. The declaration ... can only appear at the end of an argument list. Our

minprintf is declared as

void minprintf(char *fmt, ... )

since we will not return the character count that print£ does.

The tricky bit is how minprintf walks along the argument list when the list

doesn't even have a name. The standard header <stdarg. h> contains a set of

macro definitions that define how to step through an argument list. The

implementation of this header will vary from machine to machine, but the inter-

face it presents is uniform.

The type va_list is used to declare a variable that will refer to each argu-ment

in turn; in minprintf, this variable is called ap, for "argument pointer." The macro

va_start initializes ap to point to the first unnamed argument. It must be called

once before ap is used. There must be at least one named argu-ment; the final

named argument is used by va_start to get started.

156

INPUT AND OUTPUT

CHAPTER 7

Each call of va_arg returns one argument and steps ap to the next;

va_a:rg uses a type name to determine what type to return and how big a step to

take. Finally, va_end does whatever cleanup is necessary. It must be called before

the function returns.

These properties form the basis of our simplified print£:

#include <stdarq.h>

I* minprintf: minimal printf with variable argument list */ void minprintf(char

*fmt, ... )

{

va_list ap; /* points to each unnamed arq in turn */

char *P, *Sval;

int ival;

double dval;

va_start(ap, fmt); /*make appoint to 1st unnamed arq */ for (p = fmt; *P; p++)

{

if (*P I= '%') {

putchar( *P);

continue;

}

switch ( *++p) {

case 'd':

ival = va_arq(ap, int);

printf ( 11 %d 11 , i val) ;

break;

case 'f':

dval = va_arq(ap, double);

printf ( 11 %f 11 , dval) ;

break;

case 's':

for (sval = va_arq(ap, char*); *SVal; sval++)

putchar ( *SVal) ;

break;

default:

put char ( *P) ;

break;

}

va_end(ap); I* clean up when done */

}

Exercise 7-3. Revise minprintf to handle more of the other facilities of

print£. D

SECTION 7.4 FORMA TIED INPUT -SCANF 157

7.4 Formatted Input-Scant

The function scan£ is the input analog of print£, providing many of the same

conversion facilities in the opposite direction.

int scanf(char *format, ... )

scan£ reads characters from the standard input, interprets them according to the

specification in format, and stores the results through the remaining argu-ments. The

format argument is described below; the other arguments, each of which must be a

pointer, indicate where the corresponding converted input should be stored. As with

print£, this section is a summary of the most useful features, not an exhaustive list.

scan£ stops when it exhausts its format string, or when some input fails to match

the control specification. It returns as its value the number of success-fully matched

and assigned input items. This can be used to decide how many items were found.

On end of file, EOF is returned; note that this is different from 0, which means that

the next input character does not match the first specification in the format string.

The next call to scan£ resumes searching immediately after the last character already

converted.

There is also a function sscan£ that reads from a string instead of the standard

input:

int sscanf (char *String, char *format, arg

, arg

, ... )

It scans the string according to the format in format, and stores the result-ing values

through argb arg

, etc. These arguments must be pointers.

The format string usually contains conversion specifications, which are used

to control conversion of input. The format string may contain:

• Blanks or tabs, which are ignored.

• Ordinary characters (not "), which are expected to match the next non-white space

character of the input stream.

• Conversion specifications, consisting of the character %, an optional assignment

suppression character *• an optional number specifying a maximum field width, an

optional h, 1, or L indicating the width of the target, and a conversion character.

A conversion specification directs the conversion of the next input field. Nor-

mally the result is placed in the variable pointed to by the corresponding argu-

ment. If assignment suppression is indicated by the * character, however, the

input field is skipped; no assignment is made. An input field is defined as a

string of non-white space characters; it extends either to the next white space

character or until the field width, if specified, is exhausted. This implies that

scan£ will read across line boundaries to find its input, since newlines are white

space. (White space characters are blank, tab, newline, carriage return, vertical tab,

an4 formfeed.)

The conversion character indicates the interpretation of the input field. The

corresponding argument must be a pointer, as required by the call-by-value

158 INPUT AND OUTPUT CHAPTER 7

semantics of C. Conversion characters are shown in Table 7-2.

TABLE 7-2. BASIC SCANF CONVERSIONS

CHARACTER

INPUT DATA; ARGUMENT TYPE

d decimal integer; int *·

i integer; int *· The integer may be in octal (leading O) or

hexadecimal (leading Ox or ox).

o octal integer (with or without leading zero); int *·

uunsigned decimal integer; unsigned int *·

xhexadecimal integer (with or without leading Ox or ox); int *·

ccharacters; char *· The next input characters (default I) are placed at the

indicated spot. The normal skip over white space is suppressed;

to read the next non-white space character, use

%1s.

s character string (not quoted); char *• pointing to an array of

characters large enough for the string and a terminating ''\0' that

will be added.

e, f, g floating-point number with optional sign, optional decimal point

and optional exponent; float *·

literal %; no assignment is made.

The conversion characters d, i, o, u, and x may be preceded by h to indi-cate

that a pointer to short rather than int appears in the argument list, or by 1 (letter

ell) to indicate that a pointer to long appears in the argument list. Similarly, the

conversion characters e, f, and g may be preceded by 1 to indi-cate that a pointer

to double rather than float is in the argument list.

As a first example, the rudimentary calculator of Chapter 4 can be written

with scan£ to do the input conversion:

#include <stdio.h>

main() /*rudimentary calculator*/

{

double sum, v;

sum = 0;

while ( scanf ( "%lf" , &v) == 1 )

printf("'\t%.2f'\n", sum+= v);

return 0;

}

Suppose we want to read input lines that contain dates of the form

25 Dec 1988

The scan£ statement is

SECTION 7.4

FORMA TIED INPUT -SCANF

159

int day, year;

char monthname[20];

scan£ ( "%d %s %d" , &day, monthname,

&year) ;

No&. is used with monthname, since an array name is a pointer.

Literal characters can appear in the scanf format string; they must match the

same characters in the input. So we could read dates of the form mm/dd/yy with

this scanf statement:

int day, month, year;

scanf("%d/%d/%d", &month, &day, &year);

scanf ignores blanks and tabs in its format string. Furthermore, it skips over

white space (blanks, tabs, newlines, etc.) as it looks for input values. To read input

whose format is not fixed, it is often best to read a line at a time, then pick it apart

with sscanf. For example, suppose we want to read lines that might contain a date in

either of the forms above. Then we could write

while (getline(line, sizeof(line)) > 0) {

if (sscanf(line, "%d %s %d", &day, monthname, &year) == 3)

printf("valid: %s\n", line); /* 25 Dec 1988 form*/

else if (sscanf(line, "%d/%d/%d", &month, &day, &year) == 3)

print£( "valid: %s\n", line); I* mm/dd/yy form */

else

printf("invalid: %s\n", line); /* invalid form*/

}

Calls to scanf can be mixed with calls to other input functions. The next call

to any input function will begin by reading the first character not read by scanf.

A final warning: the arguments to scanf and sscanf must be pointers. By far

the most common error is writing

scanf ( "%d" , n) ;

instead of

scanf( "%d", &n);

This error is not generally detected at compile time.

Exercise 7-4. Write a private version of scanf analogous to minprintf from the

previous section. 0

Exercise 7-5. Rewrite the postfix calculator of Chapter 4 to use scanf and/or

sscanf to do the input and number conversion. 0

160 INPUT AND OUTPUT CHAPTER 1

7.5 File Access

The examples so far have all read the standard input and written the stand-ard

output, which are automatically defined for a program by the local operat-ing

system.

The next step is to write a program that accesses a file that is not already

connected to the program. One program that illustrates the need for such

operations is cat, which concatenates a set of named files onto the standard

output. cat is used for printing files on the screen, and as a general-purpose input

collector for programs that do not have the capability of accessing files by name.

For example, the command

cat x.c y.c

prints the contents of the files x . c and y. c (and nothing else) on the standard

output.

The question is how to arrange for the named files to be read-that is, how to

connect the external names that a user thinks of to the statements that read the

data.

The rules are simple. Before it can be read or written, a file has to be opened

by the library function f open. f open takes an external name like x • c or y. c, does

some housekeeping and negotiation with the operating system (details of which

needn't concern us), and returns a pointer to be used in subse-quent reads or writes

of the file.

This pointer, called the file pointer, points to a structure that contains infor-

mation about the file, such as the location of a buffer, the current character

position in the buffer, whether the file is being read or written, and whether

errors or end of file have occurred. Users don't need to know the details, because

the definitions obtained from < stdio . h> include a structure declara-tion called

FILE. The only declaration needed for a file pointer is exemplified by

FILE *fp;

FILE *fopen(char *name, char *mode);

This says that fp is a pointer to a FILE, and £open returns a pointer to a FILE.

Notice that FILE is a type name, like int, not a structure tag; it is defined with a

typedef. (Details of how £open can be implemented on the UNIX system are given in

Section 8.5.)

The call to in a program is

fp = fopen(name, mode);

The first argument of £open is a character string containing the name of the file.

The second argument is the mode, also a character string, which indicates how

one intends to use the file. Allowable modes include read ("r"), write ( "w"), and

append ("a"). Some systems distinguish between text and binary files; for the latter,

a "b" must be appended to the mode string.

SECTION 7.5 FILE ACCESS 161

If a file that does not exist is opened for writing or appending, it is created if

possible. Opening an existing file for writing causes the old contents to be dis-

carded, while opening for appending preserves them. Trying to read a file that does

not exist is an error, and there may be other causes of error as well, like trying to

read a file when you don't have permission. If there is any error, fopen will return

NULL. (The error can be identified more precisely; see the discussion of error-

handling functions at the end of Section 1 in Appendix B.)

The next thing needed is a way to read or write the file once it is open. There

are several possibilities, of which qetc and putc are the simplest. qetc returns the

next character from a file; it needs the file pointer to tell it which file.

int getc(FILE •fp)

qetc returns the next character from the stream referred to by fp; it returns EOF for

end of file or error.

putc is an output function:

int putc(int c, FILE •fp)

putc writes the character c

EOF if an error occurs. Like

macros instead of functions.

to the file fp and returns the character written, or

qetchar and putchar, qetc and putc may be

When a C program is started, the operating system environment is responsi-

ble for opening three files and providing file pointers for them. These files are

the standard input, the standard output, and the standard error; the correspond-

ing file pointers are called stdin, stdout, and stderr, and are declared in <stdio. h>.

Normally stdin is connected to the keyboard and stdout and stderr are connected to

the screen, but stdin and stdout may be redirected to files or pipes as described in

Section 7.1.

qetchar and putchar can be defined in terms of qetc, putc, stdin, and stdout as

follows:

#define getchar() getc(stdin)

#define putchar(c) putc((c), stdout)

For formatted input or output of files, the functions fscanf and fprintf may be

used. These are identical to scanf and print£, except that the first argument is a

file pointer that specifies the file to be read or written; the format string is the

second argument.

int fscanf(FILE •fp, char •format, ... )

int fprintf(FILE •fp, char •format, ... )

With these preliminaries out of the way, we are now in a position to write the

program cat to concatenate files. The design is one that has been found

convenient for many programs. If there are command-line arguments, they are

interpreted as filenames, and processed in order. If there are no arguments, the

standard input is processed.

162 INPUT AND OUTPUT CHAPTER 7

#include <stdio.h>

/* cat: concatenate files, version 1 */ main(int argc,

char *argv[])

{

FILE *fp;

void filecopy(FILE *t FILE*);

if (argc == 1) /*no args; copy standard input */ filecopy(stdin,

stdout);

else

while (--argc > 0)

if ( ( fp = £open( *++argv, "r")) == NULL) {

printf("cat: can't open %s\n", *argv); return 1;

} else {

filecopy(fp, stdout);

fclose(fp);

}

return 0;

}

/* filecopy: copy file ifp to file ofp */ void filecopy(FILE

*ifp, FILE *Ofp)

{

int c;

while ((c = getc(ifp)) I= EOF)

putc(c, ofp);

}

The file pointers stdin and stdout are objects of type FILE *· They are constants,

however, not variables, so it is not possible to assign to them.

The function

int fclose(FILE *fp)

is the inverse of £open; it breaks the connection between the file pointer and the

external name that was established by £open, freeing the file pointer for another

file. Since most operating systems have some limit on the number of files that a

program may have open simultaneously, it's a good idea to free file pointers

when they are no longer needed, as we did in cat. There is also another reason for

£close on an output file-it flushes the buffer in which putc is collecting output.

£close is called automatically for each open file when a program terminates

normally. (You can close stdin and stdout if they are not · needed. They can also be

reassigned by the library function £reopen.)

SECTION 7.6 ERROR HANDLING-STDERR AND EXIT 163

7.6 Error Handllng-Stderr and Exit

The treatment of errors in cat is not ideal. The trouble is that if one of the files

can't be accessed for some reason, the diagnostic is printed at the end of the

concatenated output. That might be acceptable if the output is going to a screen,

but not if it's going into a file or into another program via a pipeline.

To handle this situation better, a second output stream, called stderr, is

assigned to a program in the same way that stdin and stdout are. Output written

on stderr normally appears on the screen even if the standard output is redirected.

Let us revise cat to write its error messages on the standard error.

#include <stdio.h>

/* cat: concatenate files, version 2 */ main(int argc,

char *argv[])

{

FILE

*fp;

void filecopy(FILE

*•

FILE*);

char

*Prog = argv[O];

/* progr~m name for errors*/

(argc == 1)

args;

copy

standard input

else

filecopy(stdin,

stdout);

while (--argc > 0)

NULL)

if ( (fp

= fopen(*++argv,

"r"))

{

fprintf(stderr,

"%s: can't open %s\n",

prog,

*argv);

exit( 1);

} else

{

filecopy(fp, stdout);

fclose(fp);

}

(ferror(stdout))

{

fprintf(stderr,

"%s: error writing

stdout\n", prog);

exit( 2);

}

exit(O);

}

The program signals errors two ways. First, the diagnostic output produced

by fprintf goes onto stderr, so it finds its way to the screen instead of

disappearing down a pipeline or into an output file. We included the program

name, from argv [ 0 ] , in the message, so if this program is used with others, the

source of an error is identified.

Second, the program uses the standard library function exit, which ter-

minates program execution when it is called. The argument of exit is avail-able

to whatever process called this one, so the success or failure of the program can

be tested by another program that uses this one as a sub-process.

164 INPUT AND OUTPUT CHAPTER 7

Conventionally, a return value of 0 signals that all is well; non-zero values usu-ally

signal abnormal situations. exit calls fclose for each open output file, to flush out

any buffered output.

Within main, return expr is equivalent to exit ( expr). exit has the advantage that it

can be called from other functions, and that calls to it can be found with a pattern-

searching program like those in Chapter 5.

The function ferror returns non-zero if an error occurred on the stream

fp.

int ferror(FILE *fP)

Although output errors are rare, they do occur (for example, if a disk fills up), so a

production program should check this as well.

The function feof (FILE *) is analogous to ferror; it returns non-zero if end of file

has occurred on the specified file.

int feof(FILE *fP)

We have generally not worried about exit status in our small illustrative pro-

grams, but any serious program should take care to return sensible, useful status

values.

7. 7 Line Input and Output

The standard library provides an input routine fgets that is similar to the getline

function that we have used in earlier chapters:

char *fqets(char *line, int maxline, FILE *fp)

fgets reads the next input line (including the newline) from file fp into the character

array line; at most maxline-1 characters will be read. The result-ing line is terminated

with '\0 '. Normally fgets returns line; on end of file or error it returns NULL. (Our

getline returns the line length, which is a more useful value; zero means end of file.)

For output, the function fputs writes a string (which need not contain a newline)

to a file:

int fputs(char *line, FILE *fp)

It returns EOF if an error occurs, and zero otherwise.

The library functions gets and puts are similar to fgets and fputs, but operate on

stdin and stdout. Confusingly, gets deletes the terminal '\n ', and puts adds it.

To show that there is nothing special about functions like fgets and fputs, here

they are, copied from the standard library on our system:

SECTION 7.7 LINE INPUT AND OUTPUT 165

/* fgets: get at most n chars from iop */

char *fgets(char

*S, int

n, FILE

*iop)

{

int

char

*CS;

cs = s;

while (--n > 0 && (c

= getc(iop)) I=

EOF)

if ((*CS++ =c) == '\n')

break;

*CS =

'\0';

EOF &&

return

cs ==

? NULL

}

/* fputs: put string s on file iop *I

int fputs(char *S, FILE

dop)

{

int c;

while

= *S++)

putc(c, iop);

return

ferror(iop) ?

EOF

}

The standard specifies that ferror returns non-zero for error; fputs returns EOF for

error and a non-negative value otherwise.

It is easy to implement our getline from fgets:

/* getline: read a line, return length */ int getline(char

*line, int max)

{

if (fgets(line, max, stdin) == NULL)

return 0;

else

return strlen(line);

}

Exercise 7-6. Write a program to compare two files, printing the first line where

they differ. 0

Exercise 7-7. Modify the pattern finding program of Chapter 5 to take its input from

a set of named files or, if no files are named as arguments, from the stand-ard input.

Should the file name be printed when a matching line is found? 0

Exerci~ 7-8. Write a program to print a set of files, starting each new one on a new

page, with a title and a running page countfor each file. D

166 INPUT AND OUTPUT CHAPTER 7

7.8 Miscellaneous Functions

The standard library provides a wide variety of functions. This section is a

brief synopsis of the most useful. More details and many other functions can be

found in Appendix B.

7 .8. 1 String Operations

We have already mentioned the string functions strlen, strcpy, strcat, and strcmp,

found in <string. h>. In the following, s and t are char *'s, and c and n are ints.

strcat(s,t) concatenate t to end of s

strncat(s,t,n) concatenate n characters of t to end of s

strcmp(s,t) return negative, zero, or positive for

s < t, s == t, or s > t

strncmp(s,t,n)same as strcmp but only in first n characters

strcpy(s,t) copy t to s

strncpy(s,t,n) copy at most n characters of t to s

strlen(s) return length of s

stfbhr(s,c) return pointer to first c in s, or NULL if not present

strrchr(s,c) return pointer to last c in s, or NULL if not present

7 .8.2 Character Class Testing and Conversion

Several functions from <ctype. h> perform character tests and conversions. In

the following, c is an int that can be represented as an unsigned char, or EOF. The

functions return int.

isalpha(c) non-zero if c is alphabetic, 0 if not

isupper(c) non-zero if c is upper case, 0 if not

islower(c) non-zero if c is lower case, 0 if not

isdigit(c) non-zero if c is digit, 0 if not

isalnum(c) non-zero if isalpha (c) or isdigi t (c), 0 if not

isspace(c) non-zero if c is blank, tab, newline, return, formfeed, vertical tab

toupper(c) return c converted to upper case

tolower(c) return c converted to lower case

7 .8.3 Ungetc

The standard library provides a rather restricted version of the function

ungetch that we wrote in Chapter 4; it is called ungetc.

int ungetc(int c, FILE *fp)

pushes the character c back onto file fp, and returns either c, or EOF for an error.

Only one character of pushback is guaranteed per file. ungetc may be used with

any of the input functions like scan£, getc, or getchar.

SECTION 7.8 MISCELLANEOUS FUNCTIONS 167

7 .8.4 Command Execution

The function system{ char *S) executes the command contained in the character

string s, then resumes execution of the current program. The con-tents of s depend

strongly on the local operating system. As a trivial example, on UNIX systems, the

statement

system( "date");

causes the program date

standard output. system

command executed. In the

by exit.

to be run; it prints the date and time of day on the

returns a system-dependent integer status from the

UNIX system, the status return is the value returned

7 .8.5 Storage Management

The functions malloc and calloc obtain blocks of memory dynamically.

void *malloc(size_t n)

returns a pointer to n bytes of uninitialized storage, or NULL if the request can-not

be satisfied.

void *Calloc(size_t n, size_t size)

returns a pointer to enough space for an array of n objects of the specified size, or

NULL if the request cannot be satisfied. The storage is initialized to zero.

The pointer returned by malloc or calloc has the proper alignment for the object

in question, but it must be cast into the appropriate type, as in

int dp;

ip = (int *) calloc(n, sizeof(int));

free { p) frees the space pointed to by p, where p was originally obtained by a

call to malloc or calloc. There are no restrictions on the order in which space is

freed, but it is a ghastly error to free something not obtained by calling calloc or

malloc.

It is also an error to use something after it has been freed. A typical but incorrect

piece of code is this loop that frees items from a list:

for (p = head; p I= NULL; p = p->next) I* WRONG */

free(p);

The right way is to save whatever is needed before freeing:

for (p = head; p I= NULL; p = q) {

q = p->next;

free(p);

}

Section 8.7 shows the implementation of a storage allocator like malloc, in

168 INPUT AND OUTPUT CHAPTER 7

which allocated blocks may be freed in any order.

7 .8.6 Mathematical Functions

There are more than twenty mathematical functions declared in <math. h>; here are some

of the more frequently used. Each takes one or two double arguments and returns a

sin(x)

cos(x)

atan2 (y ,x)

exp(x)

loq(x)

loq10(x)

pow(x,y)

sine of x, x in radians

cosine of x, x in radians

arctangent of y /x, in radians

exponential function ex

natural (base e) logarithm of x (x > 0)

common (base 10) logarithm of x (x > 0)

sqrt(x)

fabs(x)

square root of x (x ~0)

absolute value of x

7 .8. 7 Random Number Generation

The function computes a sequence of pseudo-random integers in the

range zero to RAND_MAX, which is defined in <stdlib.h>. One way to pro-duce

random floating-point numbers greater than or equal to zero but less than one is

#define £rand() ((double) rand() I (RAND_MAX+1.0))

(If your library already provides a function for floating-point random numbers, it

is likely to have better statistical properties than this one.)

The function srand (unsigned) sets the seed for rand. The portable

implementation of rand and srand suggested by the standard appears in Sec-tion

2.7.

Exercise 7-9. Functions like isupper can be implemented to save space or to save

time. Explore both possibilities. D

cHAPTER a: The UNIX System Interface

The UNIX operating system provides its services through a set of system calls,

which are in effect functions within the operating system that may be called by

user programs. This chapter describes how to use some of the most important

system calls from C programs. If you use UNIX, this should be directly helpful,

for it is sometimes necessary to employ system calls for max· imum efficiency,

or to access some facility that is not in the library. Even if you use Con a

different operating system, however, you should be able to glean insight into C

programming from studying these examples; although details vary, similar code

will be found on any system. Since the ANSI C library is in many cases modeled

on UNIX facilities, this code may help your understanding of the library as well.

The chapter is divided into three major parts: input/output, file system, and

storage allocation. The first two parts assume a modest familiarity with the

external characteristics of UNIX systems.

Chapter 7 was concerned with an input/output interface that is uniform across

operating systems. On any particular system the routines of the standard library

have to be written in terms of the facilities provided by the host system. In the

next few sections we will describe the UNIX system calls for input and output,

and show how parts of the standard library can be implemented with them.

8. 1 File Descriptors

In the UNIX operating system, all input and output is done by reading or

writing files, because all peripheral devices, even keyboard and screen, are files

in the file system. This means that a single homogeneous interface handles all

communication between a program and peripheral devices.

In the most general case, before you read or write a file, you must inform the

system of your intent to do so, a process called opening the file. If you are going

to write on a file it may also be necessary to create it or to discard its pre-vious

contents. The system checks your right to do so (Does the file exist? Do

169

170 THE UNIX SYSTEM INTERFACE CHAPTER 8

you have permission to access it?), and if all is well, returns to the program a

small non-negative integer called a file descriptor. Whenever input or output is to

be done on the file, the file descriptor is used instead of the name to identify the

file. (A file descriptor is analogous to the file pointer used by the standard

library, or to the file handle of MS-DOS.) All information about an open file is

maintained by the system; the user program refers to the file only by the file

descriptor.

Since input and output involving keyboard and screen is so common, special

arrangements exist to make this convenient. When the command interpreter (the

"shell") runs a program, three files are open, with file descriptors 0, 1, and 2,

called the standard input, the standard output, and the standard error. If a

program reads 0 and writes 1 and 2, it can do input and output without worry-ing

about opening files.

The user of a program can redirect 1/0 to and from files with < and >:

prog <infile >outfile

In this case, the shell changes the default assignments for file descriptors 0 and 1

to the named files. Normally file descriptor 2 remains attached to the screen, so

error messages can go there. Similar observations hold for input or output

associated with a pipe. In all cases, the file assignments are changed by the shell,

not by the program. The program does not know where its input comes from nor

where its output goes, so long as it uses file 0 for input and 1 and 2 for output.

8.2 Low Leveii/0-Read and Write

Input and output uses the read and write system calls, which are accessed

from C programs through two functions called read and write. For both, the first

argument is a file descriptor. The second argument is a character array in your

program where the data is to go to or come from. The third argument is the

number of bytes to be transferred.

int n_read = read(int fd, char *buf, int n);

int n_written = write(int fd, char *buf, int n);

Each call returns a count of the number of bytes transferred. On reading, the

number of bytes returned may be less than the number requested. A return value

of zero bytes implies end of file, and -1 indicates an error of some sort. For

writing, the return value is the number of bytes written; an error has occurred if

this isn't equal to the number requested.

Any number of bytes can be read or written in one call. The most common

values are 1, which means one character at a time ("unbuffered"), and a number

like 1024 or 4096 that corresponds to a physical block size on a peri-pheral

device. Larger sizes will be more efficient because fewer system calls

SECTION 8.2 LOW LEVEL I/O-READ AND WRITE 171

will be made.

Putting these facts together, we can write a simple program to copy its input

to its output, the equivalent of the file copying program written for Chapter 1.

This program will copy anything to anything, since the input and output can be

redirected to any file or device.

#include "syscalls.h"

main()

/*copy input to output*/

{

char buf[BUFSIZ];

int n;

while ((n = read(O, buf, BUFSIZ)) > 0)

write(1, buf, n);

return 0;

}

We have collected function prototypes for the system calls into a file called

syscalls. h so we can include it in the programs of this chapter. This name is not

standard, however.

The parameter BUFSIZ is also defined in syscalls. h; its value is a good size

for the local system. If the file size is not a multiple of BUFSIZ, some read will

return a smaller number of bytes to be written by write; the next call to read after

that will return zero.

It is instructive to see how read and write can be used to construct higher-level

routines like getchar, putchar, etc. For example, here is a ver-sion of getchar that does

unbuffered input, by reading the standard input one character at a time.

#include "syscalls.h"

/* getchar: unbuffered single character input */ int getchar(void)

{

char c;

return (read(O, &c, 1) == 1) ? (unsigned char) c EOF;

}

c must be a char, because read needs a character pointer. Casting c to unsigned char

in the return statement eliminates any problem of sign exten-sion.

The second version of getchar does input in big chunks, and hands out the

characters one at a time.

172 THE UNIX SYSTEM INTERFACE CHAPTER 8

#include "syscalls.h"

/* getchar: simple buffered version */ int

getchar(void)

{

static char buf[BUFSIZ];

static char *bufp = buf;

static int n = 0;

if (n == 0) { /* buffer is empty */ n = read(O,

buf, sizeof buf); bufp = buf;

}

return (--n >= 0)

? (unsigned char)

*bufp++

EOF;

}

If these versions of qetchar were to be compiled with <stdio. h> included, it would

be necessary to #undef the name qetchar in case it is implemented as a macro.

8.3 Open, Creat, Close, Unlink

Other than the default standard input, output and error, you must explicitly

open files in order to read or write them. There are two system calls for this,

open and ere at [sicl.

open is rather like the £open discussed in Chapter 7, except that instead of

returning a file pointer, it returns a file descriptor, which is just an int. open returns -

1 if any error occurs.

#include <fcntl.h>

int fd;

int open(char •name, int flags, int perms);

fd • open(name, flags, perms);

As with £open, the name argument is a character string containing the filename.

The second argument, flaqs, is an int that specifies how the file is to be opened;

the main values are

o_RDONLY open for reading only

o_WRONLY open for writing only

o_RDWR open for both reading and writing

These constants are defined in <fcntl. h> on System V UNIX systems, and in

<sys/file. h> on Berkeley (BSD) versions.

To open an existing file for reading,

fd = open(name, O_RDONLY, 0);

SECTION 8.3 OPEN, CREAT, CLOSE, UNLINK 173

The perms argument is always zero for the uses of open that we will discuss. It is

an error to try to open a file that does not exist. The system call

creat is provided to create new files, or to re-write old ones.

int creat(char *name, int perms);

fd = creat(name, perms);

returns a file descriptor if it was able to create the file, and -1 if not. If the file

already exists, creat will truncate it to zero length, thereby discarding its previous

contents; it is not an error to creat a file that already exists.

If the file does not already exist, ereat creates it with the permissions specified

by the perms argument. In the UNIX file system, there are nine bits of permission

information associated with a file that control read, write and exe-cute access for the

owner of the file, for the owner's group, and for all others. Thus a three-digit octal

number is convenient for specifying the permissions. For example, 0755 specifies

read, write and execute permission for the owner, and read and execute permission

for the group and everyone else.

To illustrate, here is a simplified version of the UNIX program cp, which

copies one file to another. Our version copies only one file, it does not permit the

second argument to be a directory, and it invents permissions instead of copying

them.

#include <stdio.h>

#include <fcntl.h>

#include "syscalls.h"

#define PERMS 0666 /* RW for owner, group, others */

void error(char *• ... );

/* cp: copy f1 to f2 */

main(int argc, char *argv[])

{

int f1, f2, n;

char buf[BUFSIZ];

if (argc I= 3)

error ( "Usage: cp from to" ) ;

if ((f1 = open(argv(1], O_RDONLY, 0)) == -1)

error("cp: can't open "s", argv[1]); if ((f2 = creat(argv[2],

PERMS)) == -1)

error("cp: can't create "s, mode "03o", argv[ 2],

PERMS);

while ((n = read(f1, buf, BUFSIZ)) > 0)

if (write(f2, buf, n) I= n)

error("cp: write error on file "s", argv[2]); return 0;

}

This program creates the output file with fixed permissions of 0666. With the

174 THE UNIX SYSTEM INTERFACE CHAPTER 8

stat system call, described in Section 8.6, we can determine the mode of an existing

file and thus give the same mode to the cqpy.

Notice that the function error is called with variable argument lists much like

print£. The implementation of error illustrates how to use another member of the

print£ family. The standard library function vprintf is like print£ except that the

variable argument list is replaced by a single argument that has been initialized by

calling the va_start macro. Similarly, vfprintf and vsprintf match fprintf and sprint£.

#include <stdio.h>

#include <stdarg.h>

I• error: print an error message and die •I void error(char •fmt,

... )

{

va_list args;

va_start(args, fmt);

fprintf(stderr, "error: ");

vfprintf(stderr, fmt, args);

fprintf ( stderr, "\n" ) ;

va_end(args);

exit( 1);

}

There is a limit (often about 20) on the number of files that a program may

have open simultaneously. Accordingly, any program that intends to process

many files must be prepared to re-use file descriptors. The function close ( int fd)

breaks the connection between a file descriptor and an open file, and frees the file

descriptor for use with some other file; it corresponds to

£ close in the standard library except that there is no buffer to flush. Termi-nation of

a program via exit or return from the main program closes all open files.

The function unlink ( char *name ) removes the file name from the file system.

It corresponds to the standard library function remove.

Exercise 8-1. Rewrite the program cat from Chapter 7 using read, write, open and

close instead of their standard library equivalents. Perform experi-ments to

determine the relative speeds of the two versions. D

8.4 Random Access- Lseek

Input and output are normally sequential: each read or write takes place at a

position in the file right after the previous one. When necessary, however, a file

can be read or written in any arbitrary order. The system call lseek provides a

way to move around in a file without reading or writing any data:

SECTION 8.5 EXAMPLE-AN IMPLEMENTATION OF FOPEN AND GETC 175

long lseek(int fd, long offset, int origin);

sets the current position in the file whose descriptor is fd to offset, which is taken

relative to the location specified by origin. Subsequent reading or writ-ing will

begin at that position. origin can be 0, 1, or 2 to specify that offset is to be

measured from the beginning, from the current position, or from the end of the file

respectively. For example, to append to a file (the redirection > > in the UNIX shell,

or 11 a 11 for f open), seek to the end before writing:

lseek(fd, OL, 2);

To get back to the beginning ("rewind"),

lseek(fd, OL, 0);

Notice the OL argument; it could also be written as (long) 0 or just as 0 if lseek is

properly declared.

With lseek, it is possible to treat files more or less like large arrays, at the

price of slower access. For example, the following function reads any number of

bytes from any arbitrary place in a file. It returns the number read, or -1 on error.

#include "syscalls.h"

/* get: read n bytes from position pos */ int get(int fd, long

pos, char *buf, int n)

{

if (lseek(fd, pos, 0) >= 0) /* get to pos */ return read(fd, buf,

n);

else

return -1;

}

The return value from lseek is a long that gives the new position in the file, or -1 if

an error occurs. The standard library function fseek is similar to lseek except that

the first argument is a FILE * and the return is non-zero if an error occurred.

8.5 Example-An Implementation of Fopen and Gate

Let us illustrate how some of these pieces fit together by showing an imple-

mentation of the standard library routines fopen and getc.

Recall that files in the standard library are described by file pointers rather

than file descriptors. A file pointer is a pointer to a structure that contains several

pieces of information about the file: a pointer to a buffer, so the file can be read

in large chunks; a count of the number of characters left in the buffer; a pointer

to the next character position in the buffer; the file descriptor; and flags

describing read/write mode, error status, etc.

176 THE UNIX SYSTEM INTERFACE CHAPTER 8

The data structure that describes a file is contained in <stdio. h>, which must

be included (by #include) in any source file that uses routines from the standard

input/output library. It is also included by functions in that library. In the

following excerpt from a typical <stdio. h>, names that are intended for use only

by functions of the library begin with an underscore so they are less likely to

collide with names in a user's program. This convention is used by all standard

library routines.

#define

NULL

#define

EOF

( -1)

#define BUFSIZ

1024

#define OPEN_MAX

typedef struct iobuf int cnt;

char *Ptr;

char *base;

int flag;

int fd;

/* max #files open at once */

{

/* characters left */

/* next character position *I /* location

of buffer */

/* mode of file access *I /* file

descriptor */

} FILE;

extern FILE _iob[OPEN_MAX];

#de'fine stdin (&._iob[O])

#define stdout (&._iob[1])

#define stderr (&._iob[2])

enum _flags

{

READ

= 01,

WRITE

= 02,

UNBUF

= 04,

EOF

= 010,

ERR

= 020

} ;

/* file open for reading *I /* file

open for writing *I /* file is

unbuffered */

/* EOF has occurred on this file */ /* error

occurred on this file */

int _fillbuf(FILE

*);

int _flushbuf(int,

FILE*);

#define

feof(p)

(((p)->flag &. _EOF)

#define

ferror(p)

(((p)->flag &. _ERR)

#define

fileno(p)

( (p) ->fd)

#define

getc(p)

(--(p)->cnt >= 0 \

? (unsigned char) *(p)->ptr++ : _fillbuf(p)) #define

putc(x,p) (--(p)->cnt >= 0 \

? *(P)->ptr++ = (x) : _flushbuf((x),p))

#define getchar() getc(stdin)

#define putchar(x) putc((x), stdout)

The getc macro normally decrements the count, advances the pointer, and

SECTION 8.5 EXAMPLE-AN IMPLEMENTATION OF FOPEN AND GETC 177

returns the character. (Recall that a long #define is continued with a

backslash.) If the count goes negative, however, getc calls the function

_ f i llbuf to replenish the buffer, re-initialize the structure contents, and

return a character. The characters are returned unsigned, which ensures that all

characters will be positive.

Although we will not discuss any details, we have included the definition of

putc to show that it operates in much the same way as getc, calling a func-tion

_flushbuf when its buffer is full. We have also included macros for accessing the

error and end-of-file status and the file descriptor.

The function fopen can now be written. Most of fopen is concerned with

getting the file opened and positioned at the right place, and setting the flag bits

to indicate the proper state. fopen does not allocate any buffer space; this is done

by _fillbuf when the file is first read.

#include

<fcntl.h>

#include

"syscalls.h"

#define

PERMS

0666

/* RW for owner, group, others */

/* fopen:

open file,

return

file

ptr */

FILE *fopen(char *name, char

*mode)

{

int

fd;

FILE

*fp;

(*mode

I= 'r' && *mode I= 'w' && *mode I= 'a')

return

NULL;

for

(fp

= _iob; fp < _iob

+ OPEN_MAX; fp++)

((fp->flag & (_READ : _WRITE)) == 0)

break;

found

free

slot

(fp

iob + OPEN_MAX)

/* no

free

slots */

return NULL;

(*mode== 'w')

= creat(name, PERMS);

else if

(*mode== 'a') {

((fd = open(name,

O_WRONLY, 0)) == -1)

= creat(name,

PERMS);

lseek(fd, OL,

2);

} else

fd = open(name, O_RDONLY,

0);

if (fd == -1)

/* couldn't access name */

return NULL;

fp->fd = fd;

fp->cnt = 0;

fp->base = NULL;

_WRITE;

fp->flag = (*mode== 'r')

? _READ

return

fp;

}

This version of fopen does not handle all of the access mode possibilities of the

178 THE UNIX SYSTEM INTERFACE CHAPTER 8

standard, though adding them would not take much code. In particular, our fopen

does not recognize the "b" that signals binary access, since that is meaningless on

UNIX systems, nor the"+" that permits both reading and writ-ing.

The first call to getc for a particular file finds a count of zero, which forces a

call of _fillbuf. If _fillbuf finds that the file is not open for reading, it returns EOF

immediately. Otherwise, it tries to allocate a buffer (if reading is to be buffered).

Once the buffer is established, _fillbuf calls read to fill it, sets the count and

pointers, and returns the character at the beginning of the buffer. Subse-quent

calls to _fillbuf will find a buffer allocated.

#include "syscalls.h"

/* fillbuf: allocate and fill input buffer */ int _fillbuf(FILE *fp)

{

int bufsize;

if ((fp->flag&(_READI_EOFI_ERR)) I= _READ)

return EOF;

bufsize = (fp->flag & _UNBUF) ? 1 : BUFSIZ;

if (fp->base == NULL) /* no buffer yet */

if ((fp->base = (char*) malloc(bufsize)) ==NULL)

return EOF; /* can't get buffer */

fp->ptr = fp->base;

fp->cnt = read(fp->fd, fp->ptr, bufsize);

if (--fp->cnt < 0) {

if (fp->cnt == -1)

fp->flag I= _EOF;

else

fp->flag I= _ERR;

fp->cnt = 0;

return EOF;

}

return (unsigned char) *fp->ptr++;

}

The only remaining loose end is how everything gets started. The array _iob

must be defined and initialized for stdin, stdout and stderr:

FILE _iob[OPEN_MAX]

= {

/* stdin, stdout, stderr: */

{ 0,

(char*)

0, _READ, 0 },

{

(char*)

_WRITE,

{

(char *)

(char

*) 0,

_WRITE

_UNBUF, 2 }

} ;

The initialization of the flag part

read, stdout is to be written, and

of the structure shows that stdin is to be

stderr is to be written unbuffered.

Exercise 8-2. Rewrite fopen and _fillbuf with fields instead of explicit bit

SECTION 8.6 EXAMPLE-LISTING DIRECTORIES 179

operations. Compare code size and execution speed. D

Exercise 8-3. Design and write _flushbuf, £flush, and £close. D

Exercise 8-4. The standard library function

int fseek(FILE *fp, long offset, int origin)

is identical to lseek except that fp is a file pointer instead of a file descriptor and

the return value is an int status, not a position. Write £seek. Make sure that your

£seek coordinates properly with the buffering done for the other functions of the

library. D

8.6 Example-Listing Directories

A different kind of file system interaction is sometimes called for-

determining information about a file, not what it contains. A directory-listing

program such as the UNIX command ls is an example-it prints the names of files

in a directory, and, optionally, other information, such as sizes, permissions, and

so on. The MS-DOS dir command is analogous.

Since a UNIX directory is just a file, ls need only read it to retrieve the

filenames. But it is necessary to use a system call to access other information

about a file, such as its size. On other systems, a system call may be needed even

to access filenames; this is the case on MS-DOS, for instance. What we want is

provide access to the information in a relatively system-independent way, even

though the implementation may be highly system-dependent.

We will illustrate some of this by writing a program called £size. £size is a

special form of ls that prints the sizes of all files named in its command-line

argument list. If one of the files is a directory, £size applies itself recur-sively to

that directory. If there are no arguments at all, it processes the current directory.

Let us begin with a short review of UNIX file system structure. A directory is

a file that contains a list of filenames and some indication of where they are

located. The "location" is an index into another table called the "inode list." The

inode for a file is where all information about a file except its name is kept. A

directory entry generally consists of only two items, the filename and an inode

number.

Regrettably, the format and precise contents of a directory are not the same

on all versions of the system. So we will divide the task into two pieces to try to

isolate the non-portable parts. The outer level defines a structure called a Dirent

and three routines opendir, readdir, and closedir to provide system-independent

access to the name and inode number in a directory entry. We will write £size with

this interface. Then we will show how to implement these on systems that use the

same directory structure as Version 7 and System V UNIX; variants are left as

exercises.

180 THE UNIX SYSTEM INTERFACE CHAPTER 8

The Dirent structure contains the inode number and the name. The max-imum

length of a filename component is NAME_MAX, which is a system-dependent value.

opendir returns a pointer to a structure called DIR, analo-gous to FILE, which is used

by readdir and closedir. This information is collected into a file called dirent. h.

#define NAME_MAX 14 /* longest filename

component; */

/* system-dependent */

typedef struct { /* portable directory entry: */

long ino; I* inode number */

char name[NAME_MAX+1]; /* name + '\0' terminator */

} Dirent;

typedef struct { /* minimal DIR: no buffering, etc. */

int fd; /* file

descriptor for

directory */

Dirent d; /* the directory entry */

} DIR;

DIR *Opendir(char *dirname);

Dirent *readdir(DIR *dfd);

void closedir(DIR *dfd);

The system call stat takes a filename and returns all of the information in the

inode for that file, or -1 if there is an error. That is,

char *name;

struct stat stbuf;

int stat(char *• struct stat *l;

stat(name, &stbuf);

fills the structure stbuf with the inode information for the file name. The structure

describing the value returned by stat is in <sys/stat. h>, and typ-ically looks like this:

struct stat /* inode information returned by stat *I

{

dev_t st_dev;

ino_t st _ino;

short st_mode;

short st_nlink;

short st_uid;

short st_gid;

dev_t st_rdev;

off

t st_size;

time

t st_atime;

time_t st_mtime;

time

t st_ctime;

} ;

/* device of inode *I /*

inode number *I /* mode

bits *I

/* number of links to file *I /* owner's

user id *I

/* owner's group id */ /* for

special files *I

/* file size in characters *I /* time last

accessed */ /* time last modified *I

/* time inode last changed *I

Most of these values are explained by the comment fields. The types like

SECTION 8.6 EXAMPLE-LISTING DIRECTORIES 181

dev _ t and ino_ t are defined in <sys/types. h>, which must be included

too.

The st_mode entry contains a set of flags describing the file. The flag

definitions are also included in <sys/stat. h>; we need only the part that deals

with file type:

#define

S_IFMT

0160000

/* type of file:

#define

S_IFDIR

0040000

/* directory */

#define

S_IFCHR

0020000

character special *I

#define

S_IFBLK

0060000

block special

#define

S_IFREG

0100000

regular */

/* ...

Now we are ready to write the program £size. If the mode obtained from stat

indicates that a file is not a directory, then the size is at hand and can be printed

directly. If the file is a directory, however, then we have to process that directory

one file at a time; it may in turn contain sub-directories, so the process is recursive.

The main routine deals with command-line arguments; it hands each argu-

ment to the function £size.

#include

<stdio.h>

#include

<strinq.h>

#include

"syscalls.h"

#include

<fcntl.h>

/* flags for read and write */

#include

typedefs

#include

structure

returned by stat */

#include

"dirent.h"

void fsize(char *);

/* print file sizes */

main(int arqc, char **arqv)

{

if (arqc == 1) /* default: current directory */ fsize(".");

else

while (--arqc > 0)

fsize ( *++arqv) ;

return 0;

}

The function £size prints the size of the file. If the file is a directory, however,

£size first calls dirwalk to handle all the files in it. Note how the flag names

s_IFMT and S_IFDIR from <sys/stat. h> are used to decide if the file is a directory.

Parenthesization matters, because the precedence of & is lower than that of ==.

182THE UNIX SYSTEM INTERFACE

CHAPTER 8

int stat(char *•

struct stat •);

•>>;

void dirwalk(char

*• void (•fcn)(char

I• fsize: print size of file "name" •I void fsize(char

•name)

{

struct stat stbuf;

if (stat(name, &stbuf) == -1) {

fprintf(stderr, "fsize: can't access %s\n", name); return;

}

if ((stbuf.st_mode & S_IFMT) == S_IFDIR)

dirwalk(name, fsize);

printf("%8ld %s\n", stbuf.st_size, name);

}

The function dirwalk is a general routine that applies a function to each file in

a directory. It opens the directory, loops through the files in it, calling the

function on each, then closes the directory and returns. Since fsize calls dirwalk

on each directory, the two functions call each other recursively.

#define MAX.PATH 1024

I• dirwalk: apply fen to all files in dir •I void dirwalk(char

•dir, void (•fcn)(char •>>

{

char name[MAX_PATH];

Dirent •dt:>;

DIR •dfd;

if ((dfd = opendir(dir)) ==NULL) {

fprintf(stderr, "dirwalk: can't open %s\n", dir);

return;

}

while ((dp = readdir(dfd)) I= NULL) { if

(strcmp(dp->name, ".") == 0

:: strcmp(dp->name, " .• ") == 0)

continue;I• skip self and parent •I

if (strlen(dir)+strlen(dp->name)+2 > sizeof(name)) fprintf(stderr,

"dirwalk: name %sl%s too long\n",

dir , dp- >name ) ;

else {

sprintf(name, "%sl%s", dir, dp->name); (•fen)

(namf!);

}

closf!dir(dfd);

}

Each call to readdir returns a pointer to information for the next file, or

SECTION 8.6 EXAMPLE-LISTING DIRECTORIES 183

NULL when there are no files left.

itself, called 11 • 11 , and its parent, 11

will loop forever.

Each directory always contains entries for these must be skipped, or the

program

Down to this level, the code is independent of how directories are formatted.

The next step is to present minimal versions of opendir, readdir, and closedir for a

specific system. The following routines are for Version 7 and System V UNIX

systems; they use the directory information in the header <sys/dir. h>, which looks

like this:

#ifndef DIRSIZ

#define DIRSIZ 14

#endif

struct direct /* directory entry */

{

ino_t d_ino; /* inode number */

char d_name[DIRSIZ]; /*long name does not have #\0' */

} ;

Some versions of the system permit much longer names and have a more com-

plicated directory structure.

The type ino _ t is a type de£ that describes the index into the inode list. It

happens to be unsigned short on the system we use regularly, but this is not the sort

of information to embed in a program; it might be different on a different system, so

the typedef is better. A complete set of "system" types is found in <sys/types. h>.

opendir opens the directory, verifies that the file is a directory (this time by the

system call fstat, which is like stat except that it applies to a file descriptor),

allocates a directory structure, and records the information:

int fstat(int fd, struct stat*);

/* opendir: open a directory for readdir calls */ DIR *Opendir(char

*dirname)

{

int

fd;

struct stat stbuf;

DIR

*dp;

((fd = open(dirname, O_RDONLY, 0)) == -1

fstat(fd, &stbuf) == -1

(stbuf.st_mode

& S_IFMT) I= S_IFDIR

(dp = (DIR *)

malloc(sizeof(DIR))) ==NULL)

return NULL;

dp->fd = fd;

return dp;

}

closedir closes the directory file and frees the space:

184 THE UNIX SYSTEM INTERFACE CHAPTER 8

I* closedir: close directory opened by opendir */ void closedir(DIR

*dp)

{

if (dp) {

close(dp->fd);

free(dp);

}

Finally, readdir uses read to read each directory entry. If a directory slot is not

currently in use (because a file has been removed), the inode number is zero, and

this position is skipped. Otherwise, the inode number and name are placed in a

static structure and a pointer to that is returned to the user. Each call overwrites

the information from the previous one.

#include <sys/dir.h> /* local directory structure */

I* readdir: read directory entries in sequence */ Dirent *readdir(DIR

*dp)

{

struct direct dirbuf; /* local directory structure */ static Dirent d; /* return:

portable structure */

while (read(dp->fd, (char*) &dirbuf, sizeof(dirbuf))

== sizeof(dirbuf)) {

if (dirbuf.d_ino == 0) /* slot not in use */ continue;

d.ino = dirbuf.d_ino;

strncpy(d.name, dirbuf.d_name, DIRSIZ); d.name[DIRSIZ] =

'\0'; /*ensure termination*/ return &d;

}

return NULL;

}

Although the fsize program is rather specialized, it does illustrate a couple of

important ideas. First, many programs are not "system programs"; they merely

use information that is maintained by the operating system. For such programs, it

is crucial that the representation of the information appear only in standard

headers, and that programs include those files instead of embedding the

declarations in themselves. The second observation is that with care it is possible

to create an interface to system-dependent objects that is itself rela-tively

system-independent. The functions of the standard library are good examples.

Exercise 8-5. Modify the fsize program to print the other information con-tained

in the inode entry. D

SECTION 8.7 EXAMPLE-A STORAGE ALLOCATOR 185

8.7 Example-A Storage Allocator

In Chapter 5, we presented a very limited stack-oriented storage allocator.

The version that we will now write is unrestricted. Calls to malloc and free may

occur in any order; malloc calls upon the operating system to obtain more

memory as necessary. These routines illustrate some of the considerations

involved in writing machine-dependent code in a relatively machine-independent

way, and also show a real-life application of structures, unions and typedef.

Rather than allocating from a compiled-in fixed-sized array, malloc will

request space from the operating system as needed. Since other activities in the

program may also request space without calling this allocator, the space that

malloc manages may not be contiguous. Thus its free storage is kept as a list of free

blocks. Each block contains a size, a pointer to the next block, and the space itself.

The blocks are kept in order of increasing storage address, and the last block

(highest address) points to the first.

free list~

I . 1Vfr=1 . 1· Jl I · T4'crl

.___ _.I free, owned by malloc

Iin use I in use, owned by malloc

1: ::::::I not owned by malloc

When a request is made, the free list is scanned until a big-enough block is

found. This algorithm is called "first fit," by contrast with "best fit," which looks

for the smallest block that will satisfy the request. If the block is exactly the size

requested it is unlinked from the list and returned to the user. If the block is too

big, it is split, and the proper amount is returned to the user while the residue

remains on the free list. If no big-enough block is found, another large chunk is

obtained from the operating system and linked into the free list.

Freeing also causes a search of the free list, to find the proper place to insert

the block being freed. If the block being freed is adjacent to a free block on either

side, it is coalesced with it into a single bigger block, so storage does not become

too fragmented. Determining adjacency is easy because the free list is

maintained in order of increasing address.

One problem, which we alluded to in Chapter 5, is to ensure that the storage

returned by malloc is aligned properly for the objects that will be stored in it.

Although machines vary, for each machine there is a most restrictive type: if the

most restrictive type can be stored at a particular address, all other types may be

also. On some machines, the most restrictive type is a double; on others, int or

long suffices.

186 THE UNIX SYSTEM INTERFACE CHAPTER 8

A free block contains a pointer to the next block in the chain, a record of the

size of the block, and then the free space itself; the control information at the

beginning is called the "header." To simplify alignment, all blocks are multi-ples

of the header size, and the header is aligned properly. This is achieved by a

union that contains the desired header structure and an instance of the most

restrictive alignment type, which we have arbitrarily made a long:

typedef long Align;

/* for alignment to long boundary */

union header { /* block header: */

struct {

union header *Ptr; /* next block if on free list */

unsigned size; /* size of this block */

} s;

Align x; /* force alignment of blocks */

} ;

typedef union header Header;

The Align field is never used; it just forces each header to be aligned on a worst-

case boundary.

In malloc, the requested size in characters is rounded up to the proper number

of header-sized units; the block that will be allocated contains one more unit, for

the header itself, and this is the value recorded in the size field of the header. The

pointer returned by malloc points at the free space, not at the header itself. The

user can do anything with the space requested, but if any-thing is written outside

of the allocated space the list is likely to be scrambled.

--- · points to next free block

I tf.. l

L___ address returned to user

A block returned by malloc

The size field is necessary because the blocks controlled by malloc need not be

contiguous-it is not possible to compute sizes by pointer arithmetic.

The variable base is used to get started. If freep is NULL, as it is at the first call

of malloc, then a degenerate free list is created; it contains one block of size zero,

and points to itself. In any case, the free list is then searched. The search for a

free block of adequate size begins at the point (freep) where the last block was

found; this strategy helps keep the list homogeneous. If a too-big block is found,

the tail end is returned to the user; in this way the header of the original needs

only to have its size adjusted. In all cases, the pointer returned to the user points

to the free space within the block, which begins one unit beyond the header.

SECTION 8.7 EXAMPLE-A STORAGE ALLOCATOR 187

static Header base; I* static Header

*freep = NULL;

empty list to get started */ /* start

of free list */

I* malloc: general-purpose storage allocator */ void

*malloc(unsigned nbytes)

{

Header *P• *Prevp;

Header *morecore(unsigned);

unsigned nunits;

nunits = (nbytes+sizeof(Header)-1)/sizeof(Header) + 1; if ((prevp = freep)

==NULL) { /*no free list yet*/

base.s.ptr = freep = prevp = &base; base.s.size

= 0;

}

for (p = prevp->s.ptr; ; prevp = p, p = p->s.ptr) {

if (p->s.size >= nunits) { /* big enough */

if (p->s.size == nunits) I* exactly */

prevp->s.ptr = p->s.ptr;

else { /* allocate tail end */

p->s.size -= nunits;

p += p->s.size;

p->s.size = nunits;

}

freep = prevp;

return (void *)(p+1);

}

if (p == freep) /* wrapped around free list */ if ((p =

morecore(nunits)) ==NULL) return NULL; /* none left */

}

The function morecore obtains storage from the operating system. The details

of how it does this vary from system to system. Since asking the system for

memory is a comparatively expensive operation, we don't want to do that on

every call to malloc, so morecore requests at least NALLOC units; this larger

block will be chopped up as needed. After setting the size field, morecore inserts

the additional memory into the arena by calling free.

The UNIX system call sbrk ( n) returns a pointer to n more bytes of storage.

sbrk returns -1 if there was no space, even though NULL would have been a

better design. The - 1 must be cast to char * so it can be compared with the return

value. Again, casts make the function relatively immune to the details of pointer

representation on different machines. There is still one assumption, however,

that pointers to different blocks returned by sbrk can be meaningfully compared.

This is not guaranteed by the standard, which permits pointer comparisons only

within an array. Thus this version of malloc is port-able only among machines

for which general pointer comparison is meaningful.

188 THE UNIX SYSTEM INTERFACE CHAPTER 8

#define NALLOC 1024 /* minimum #units to request */

/* morecore: ask system for more memory */ static Header

*morecore(unsigned nu)

{

char *CP, *Sbrk(int);

Header *UP;

if (nu < NALLOC)

nu = NALLOC;

cp = sbrk(nu * sizeof(Header));

if (cp == (char *) -1) /*no space at all */ return NULL;

up = (Header *) cp;

up->s.size = nu;

free((void *)(up+1));

return freep;

}

free itself is the last thing. It scans the free list, starting at freep, look-ing for the

place to insert the free block. This is either between two existing blocks or at one

end of the list. In any case, if the block being freed is adjacent to either neighbor, the

adjacent blocks are combined. The only troubles are keeping the pointers pointing to

the right things and the sizes correct.

/* free: put block ap in free list */ void free(void *ap)

{

Header

*bp, *P;

bp = (Header *)ap- 1;

/* point to block header */

for (p

= freep;

l(bp > p

&& bp <

p->s.ptr); p = p->s.ptr)

(p >= p->s.ptr && (bp > p

I I bp < p->s.ptr))

break;

/* freed

block at

start or end of arena */

(bp

+ bp->s.size == p->s.ptr)

{ /* join to upper nbr */

bp->s.size += p->s.ptr->s.size;

bp->s.ptr = p->s.ptr->s.ptr;

}

else

bp->s.ptr = p->s.ptr;

+ p->s.size == bp)

{

/* join to lower nbr */

p->s.size += bp->s.size;

p->s.ptr = bp->s.ptr;

}

else

p->s.ptr = bp;

freep

= p;

}

Although storage allocation is intrinsically machine-dependent, the code

above illustrates how the machine dependencies can be controlled and confined

to a very small part of the program. The use of typedef and union handles

SECTION 8.7 EXAMPLE-A STORAGE ALLOCATOR 189

alignment (given that sbrk supplies an that

pointer conversions are made explicit,

system interface. Even though the details

the general approach is applicable to other

appropriate pointer). Casts arrange

and even cope with a badly-designed

here are related to storage allocation,

situations as well.

Exercise 8-6. The standard library function calloc ( n, size) returns a poiHter to n

objects of size size, with the storage initialized to zero. Write calloc, by calling

malloc or by modifying it. 0

Exercise 8-7. malloc accepts a size request without checking its plausibility; free

believes that the block it is asked to free contains a valid size field. Improve these

routines so they take more pains with error checking. 0

Exercise 8-8 Write a routine bfree ( p, n) that will free an arbitrary block p of n

characters into the free list maintained by malloc and free. By using bfree, a user

can add a static or external array to the free list at any time. 0

APPENDix A: Reference Manual

A 1. Introduction

This manual describes the C language specified by the draft submitted to ANSI on

31 October, 1988, for approval as "American National Standard for Information Systems-

Programming Language C, X3.159·1989." The manual is an interpretation of the proposed

standard, not the Standard itself, although care has been taken to make it a reliable guide to

the language.

For the most part, this document follows the broad outline of the Standard, which in

turn follows that of the first edition of this book, although the organization differs in detail.

Except for renaming a few productions, and not formalizing the definitions of the lexical

tokens or the preprocessor, the grammar given here for the language proper is equivalent to

that of the Standard.

Throughout this manual, commentary material is indented and written in smaller

type, as this is. Most often these comments highlight ways in which ANSI Standard C

differs from the language defined by the first edition of this book, or from refinements

subsequently introduced in various compilers.

A2. Lexical Conventions

A program consists of one or more translation units stored in files. It is translated in

several phases, which are described in §A 12. The first phases do low-level lexical

transformations, carry out directives introduced by lines beginning with the # character, and

perform macro definition and expansion. When the preprocessing of §A 12 is com-plete,

the program has been reduced to a sequence of tokens.

A2.1 Tokens

There are six classes of tokens: identifiers, keywords, constants, string literals, opera-

tors, and other separators. Blanks, horizontal and vertical tabs, newlines, formfeeds, and

comments as described below (collectively, "white space") are ignored except as they

separate tokens. Some white space is required to separate otherwise adjacent identifiers,

keywords, and constants.

191

192 REFERENCE MANUAL APPENDIX A

If the input stream has been separated into tokens up to a given character, the next token

is the longest string of characters that could constitute a token.

A2.2 Comments

The characters /* introduce a comment, which terminates with the characters */.

Comments do not nest, and they do not occur within string or character literals.

A2.3 Identifiers

An identifier is a sequence of letters and digits. The first character must be a letter;

the underscore _ counts as a letter. Upper and lower case letters are different. Identi-fiers

may have any length, and for internal identifiers, at least the first 31 characters are

significant; some implementations may make more characters significant. Internal iden-

tifiers include preprocessor macro names and all other names that do not have external

linkage (§All.2). Identifiers with external linkage are more restricted: implementations '

may make as few as the first six characters as significant, and may ignore case distinc-

tions.

A2.4 Keywords

The following identifiers are reserved for use as keywords, and may not be used

otherwise:

auto

double

int

struct

break

else

long

switch

case

enu.m

typedef

char

extern

return

union

const

float

short

unsigned

continue

for

signed

void

default

go to

sizeof

volatile

static

while

Some implementations also reserve the words fortran and asm.

The keywords const, signed, and volatile are new with the ANSI stand-ard; enum and

void are new since the first edition, but in common use; entry, formerly reserved but

never used, is no longer reserved.

A2.5 Constants

There are several kinds of constants. Each has a data type; §A4.2 discusses the basic

types.

constant:

integer-constant

character-constant

floating-constant

enumeration-constant

SECTION A2 LEXICAL CONVENTIONS 193

A2.5. 1 Integer Constants

An integer constant consisting of a sequence of digits is taken to be octal if it begins with 0

(digit zero), decimal otherwise. Octal constants do not contain the digits 8 or 9. A sequence of

digits preceded by Ox or OX (digit zero) is taken to be a hexadecimal integer. The hexadecimal

digits include a or A through f or F with values 10 through 15.

An integer constant may be suffixed by the letter u or U, to specify that it is unsigned. It may

also be suffixed by the letter 1 or L to specify that it is long.

The type of an integer constant depends on its form, value and suffix. (See §A4 for a

discussion of types.) If it is unsuffixed and decimal, it has the first of these types in which its

value can be represented: int, long int, unsigned long int. If it is unsuffixed octal or hexadecimal,

it has the first possible of these types: int, unsigned int, long int, unsigned long int. If it is

suffixed by u or U, then unsigned int, unsigned long int. If it is suffixed by 1 or L, then long int,

unsigned long int.

The elaboration of the types of integer constants goes considerably beyond the first

edition, which merely caused large integer constants to be long. The u suffixes are

new.

A2.5.2 Character Constants

A character constant is a sequence of one or more characters enclosed in single quotes, as in

'x'. The value of a character constant with only one character is the numeric value of the

character in the machine's character set at execution time. The value of a multi-character

constant is implementation-defined.

Character constants do not contain the ' character or newlines; in order to represent them,

and certain other characters, the following escape sequences may be used.

newline

NL (LF)

"\n

backslash

"\ "\

horizontal tab

"\ t

question mark

"\?

vertical tab

"\ v

single quote

"\'

backspace

"\b

double quote

carriage return

"\r

octal number

ooo

"\ooo

formfeed

"\f

hex number

"\xhh

audible alert

BEL

"\a

The escape "\ooo consists of the backslash followed by 1, 2, or 3 octal digits, which are taken to

specify the value of the desired character. A common example of this construc-tion is "\0 (not

followed by a digit), which specifies the character NUL. The escape "\xhh consists of the

backslash, followed by x, followed by hexadecimal digits, which are taken to specify the value

of the desired character. There is no limit on the number of digits, but the behavior is undefined

if the resulting character value exceeds that of the larges't character. For either octal or

hexadecimal escape characters, if the implementa-tion treats the char type as signed, the value is

sign-extended as if cast to char type. If the character following the "\ is not one of those specified,

the behavior is undefined.

In some implementations, there is an extended set of characters that cannot be represented in

the char type. A constant in this extended set is written with a preced-ing L, for example L 'x ',

and is called a wide character constant. Such a constant has type wchar _ t, an integral type

defined in the standard header <stddef. h>. As with

194 REFERENCE MANUAL APPENDIX A

ordinary character constants, octal or hexadecimal escapes may be used; the effect is

undefined if the specified value exceeds that representable with wchar _ t.

Some of these escape sequences are new, in particular the hexadecimal charac-ter

representation. Extended characters are also new. The character sets com-monly used

in the Americas and western Europe can be encoded to fit in the char type; the main

intent in adding wchar _ t was to accommodate Asian languages.

A2.5.3 Floating Constants

A floating constant consists of an integer part, a decimal point, a fraction part, an e or E,

an optionally signed integer exponent and an optional type suffix, one of f, F, 1, or

L. The integer and fraction parts both consist of a sequence of digits. Either the integer part

or the fraction part (not both) may be missing; either the decimal point or the e and the

exponent (not both) may be missing. The type is determined by the suffix; F or f makes it

float, Lor 1 makes it long double; otherwise it is double.

Suffixes on floating constants are new.

A2.5.4 Enumeration Constants

Identifiers declared as enumerators (see §A8.4) are constants of type int.

A2.8 String Literals

A string literal, also called a string constant, is a sequence of characters surrounded by

double quotes, as in 11 ••• 11 • A string has type "array of characters" and storage class static (see

§A4 below) and is initialized with the given characters. Whether identical string literals are

distinct is implementation-defined, and the behavior of a pro-gram that attempts to alter a

string literal is undefined.

Adjacent string literals are concatenated into a single string. After any concatena-tion, a

null byte \0 is appended to the string so that programs that scan the string can find its end.

String literals do not contain newline or double-quote characters; in order to represent them,

the same escape sequences as for character constants are available.

As with character constants, string literals in an extended character set are written with a

preceding L, as in L 11 ••• 11 • Wide-character string literals have type "array of wchar _ t."

Concatenation of ordinary and wide string literals is undefined.

The specification that string literals need not be distinct, and the prohibition against

modifying them, are new in the ANSI standard, as is the concatenation of adjacent

string literals. Wide-character string literals are new.

A3. Syntax Notation

In the syntax notation used in this manual, syntactic categories are indicated by italic

type, and literal words and characters in typewriter style. Alternative categories are usually

listed on separate lines; in a few cases, a long set of narrow alter-natives is presented on one

line, marked by the phrase "one of." An optional terminal or nonterminal symbol carries the

subscript "opt," so that, for example,

SECTION A4 MEANING OF IDENTIFIERS 195

{ expressionopt }

means an optional expression, enclosed in braces. The syntax is summarized in §A13.

Unlike the grammar given in the first edition of this book, the one given here makes

precedence and associativity of expression operators explicit.

A4. Meaning of Identifiers

Identifiers, or names, refer to a variety of things: functions; tags of structures, unions,

and enumerations; members of structures or unions; enumeration constants; typedef names;

and objects. An object, sometimes called a variable, is a location in storage, and its

interpretation depends on two main attributes: its storage class and its type. The storage class

determines the lifetime of the storage associated with the identi-fied object; the type

determines the meaning of the values found in the identified object. A name also has a

scope, which is the region of the program in which it is known, and a linkage, which

determines whether the same name in another scope refers to the same object or function.

Scope and linkage are discussed in §A 11.

A4. 1 Storage Class

There are two storage classes: automatic and static. Several keywords, together with the

context of an object's declaration, specify its storage class. Automatic objects are local to a

block (§A9.3), and are discarded on exit from the block. Declarations within a block create

automatic objects if no storage class specification is mentioned, or if the auto specifier is

used. Objects declared registE;!r are automatic, anq are (if possible) stored in fast registers

of the machine.

Static objects may be local to a block or external to all blocks, but in either case retain

their values across exit from and reentry to functions and blocks. Within a block, including

a block that provides the code for a function, static objects are declared with the keyword

static. The objects declared outside all blocks, at the same level as func-tion definitions, are

always static. They may be made local to a particular translation unit by use of the static

keyword; this gives them internal linkage. They become glo-bal to an entire program by

omitting an explicit storage class, or by using the keyword extern; this gives them external

linkage.

A4.2 Basic Types

There are several fundamental types. The standard header < 1 imi ts . h> described in

Appendix B defines the largest and smallest values of each type in the local imple-

mentation. The numbers given in Appendix B show the smallest acceptable magnitudes.

Objects declared as characters (char) are large enough to store any member of the

execution character set. If a genuine character from that set is stored in a char object, its

value is equivalent to the integer code for the character, and is non-negative. Other

quantities may be stored into char variables, but the available range of values, and

especially whether the value is signed, is implementation-dependent.

Unsigned characters declared unsigned char consume the same amount of space as plain

characters, but always appear non-negative; explicitly signed characters declared signed

char likewise take the same space as plain characters.

196

REFERENCE MANUAL

APPENDIX A

unsigned char type does not appear in the first edition of this book,

common use. signed char is new.

but is in

Besides the char types, up to three sizes of integer, declared short int, int, and

long int, are available. Plain int objects have the natural size suggested by the host machine

architecture; the other sizes are provided to meet special needs. Longer integers provide at

least as much storage as shorter ones, but the implementation may make plain integers

equivalent to either short integers, or long integers. The int types all represent signed values

unless specified otherwise.

Unsigned integers, declared using the keyword unsigned, obey the laws of arith-metic

modulo 2" where n is the number of bits in the representation, and thus arithmetic on

unsigned quantities can never overflow. The set of non-negative values that can be stored in

a signed object is a subset of the values that can be stored in the corresponding unsigned

object, and the representation for the overlapping values is the same.

Any of single precision floating point (float), double precision floating point (double),

and extra precision floating point (long double) may be synonymous, but the ones later in the

list are at least as precise as those before.

long double is new. The first edition made long float equivalent to double; the

locution has been withdrawn.

Enumerations are unique types that have integral values; associated with each

enumeration is a set of named consta;ts (§A8.4). Enumerations behave like integers, but it is

common for a compiler to issue a warning when an object of a particular enumeration type

is assigned something other than one of its constants, or an expression of its type.

Because objects of these types can be interpreted as numbers, they will be referred to as

arithmetic types. Types char, and int of all sizes, each with or without sign, and also

enumeration types, will collectively be called integral types. The types float, double, and

long double will be called floating types.

The void type specifies an empty set of values. It is used as the type returned by

functions that generate no value.

A4.3 Derived Types

Besides the basic types, there is a conceptually infinite class of derived types con-

structed from the fundamental types in the following ways:

arrays of objects. of a given type;

functions returning objects of a given type;

pointers to objects of a given type;

structures containing a sequence of objects of various types;

unions capable of containing any one of several objects of various types.

In general these methods of constructing objects can be applied recursively.

A4.4 Type Qualifiers

An object's type may have additional qualifiers. Declaring an object const announces

that its value will not be changed; declaring it volatile announces that it has special

properties relevant to optimization. Neither qualifier affects the range of values or arithmetic

properties of the object. Qualifiers are discussed in §A8.2.

SECTION A6 CONVERSIONS 197

AS. Objects and Lvalues

An object is a named region of storage; an /value is an expression referring to an object.

An obvious example of an lvalue expression is an identifier with suitable type and storage

class. There are operators that yield lvalues: for example, if E is an expres-sion of pointer

type, then *E is an lvalue expression referring to the object to which E points. The name

"lvalue" comes from the assignment expression E 1 = E2 in which the left operand E 1 must

be an lvalue expression. The discussion of each operator speci-fies whether it expects lvalue

operands and whether it yields an lvalue.

A6. Conversions

Some operators may, depending on their operands, cause conversion of the value of an

operand from one type to another. This section explains the result to be expected from such

conversions. §A6.5 summarizes the conversions demanded by most ordinary operators; it

will be supplemented as required by the discussion of each operator.

A6. 1 Integral Promotion

A character, a short integer, or an integer bit-field, all either signed or not, or an object

of enumeration type, may be used in an expression wherever an integer may be used. If an

int can represent all the values of the original type, then the value is con-verted to int;

otherwise the value is converted to unsigned int. This process is called integral promotion.

A6.2 Integral Conversions

Any integer is converted to a given unsigned type by finding the smallest non-negative

value that is congruent to that integer, modulo one more than the largest value that can be

represented in the unsigned type. In a two's complement representation, this is equivalent to

left-truncation if the bit pattern of the unsigned type is narrower, and to zero-filling

unsigned values and sign-extending signed values if the unsigned type is wider.

When any integer is converted to a signed type, the value is unchanged if it can be

represented in the new type and is implementation-defined otherwise.

A6.3 Integer and Floating

When a value of floating type is converted to integral type, the fractional part is dis-

carded; if the resulting value cannot be represented in the integral type, the behavior is

undefined. In particular, the result of converting negative floating values to unsigned

integral types is not specified.

When a value of integral type is converted to floating, and the value is in the

representable range but is not exactly representable, then the result may be either the next

higher or next lower representable value. If the result is out of range, the behavior is

undefined.

198 REFERENCE MANUAL APPENDIX A

A6.4 Floating Types

When a less precise floating value is converted to an equally or more precise floating

type, the value is unchanged. When a more precise floating value is converted to a less

precise floating type, and the value is within representable range, the result may be either

the next higher or the next lower representable value. If the result is out of range, the

behavior is undefined.

A6.5 Arithmetic Conversions

Many operators cause conversions and yield result types in a similar way. The effect is

to bring operands into a common type, which is also the type of the result. This pat-tern is

called the usual arithmetic conversions.

First, if either operand is long double, the other is converted to long double.

Otherwise, if either operand is double, the other is converted to double.

Otherwise, if either operand is float, the other is converted to float.

Otherwise, the integral promotions are performed on· both operands; then, if either

operand is unsigned long int, the other is converted to unsigned long int. Otherwise, if one

operand is long int and the other is unsigned int, the effect depends on whether a long int

can represent all values of an unsigned int; if so, the unsigned int operand is converted to

long int; if not, both are converted to unsigned long int.

Otherwise, if one operand is long int, the other is converted to long int. Otherwise, if

either operand is u:r;1signed int, the other is converted to unsigned

int.

Otherwise, both operands have type int.

There are two changes here. First, arithmetic on float operands may be done in single

precision, rather than double; the first edition specified that all floating arithmetic

was double precision. Second, shorter unsigned types, when com-bined with a larger

signed type, do not propagate the unsigned property to the result type; in the first

edition, the unsigned always dominated. The new rules are slightly more

complicated, but reduce somewhat the surprises that may occur when an unsigned

quantity meets signed. Unexpected results may still occur when an unsigned

expression is compared to a signed expression of the same size.

A6.6 Pointers and Integers

An expression of integral type may be added to or subtracted from a pointer; in such a

case the integral expression is converted as spe~ified in the discussion of the addition operator

(§A7.7).

Two pointers to objects of the same type, in the same array, may be subtracted; the

result is converted to an integer as specified in the discussion of the subtraction operator

(§A7.7).

An integral constant expression with value 0; or such an expression cast to type void *•

may be converted, by a cast, by assignment, or by comparison, to a .pointer of any type.

This produces a null pointer that is equal to another null pointer of the same type, but

unequal to any pointer to a function or object.

Certain other conversions involving pointers are permitted, but have implementation-

dependent aspects. They must be specified by an explicit type-conversion operator, or

SECTION A6 CONVERSIONS 199

cast (§§A7.5 and A8.8).

A pointer may be converted to an integral type large enough to hold it; the required size

is implementation-dependent. The mapping function is also implementation-dependent.

An object of integral type may be explicitly converted to a pointer. The mapping always

carries a sufficiently wide integer converted from a pointer back to the same pointer, but is

otherwise implementation-dependent.

A pointer to one type may be converted to a pointer to another type. The resulting

pointer may cause addressing exceptions if the subject pointer does not refer to an object

suitably aligned in storage. It is guaranteed that a pointer to an object may be con-verted to a

pointer to an object whose type requires less or equally strict storage align-ment and back

again without change; the notion of "alignment" is implementation-dependent, but objects

of the char types have least strict alignment requirements. As described in §A6.8, a pointer

may also be converted to type void * and back again without change.

A pointer may be converted to another pointer whose type is the same except for the

addition or removal of qualifiers (§§A4.4, A8.2) of the object type to which the pointer

refers. If qualifiers are added, the new pointer is equivalent to the old except for restric-tions

implied by the new qualifiers. If qualifiers are removed, operations on the underly-ing

object remain subject to the qualifiers in its actual declaration.

Finally, a pointer to a function may be converted to a pointer to another function type.

Calling the function specified by the converted pointer is implementation-dependent;

however, if the converted pointer is reconverted to its original type, the result is identical to

the original pointer.

A6.7 Void

The !(nonexistent) value of a void object may not be used in any way, and neither

explicit nor implicit conversion to any non-void type may be applied. Because a void

expression denotes a nonexistent value, such an expression may be used only where the

value is not required, for example as an expression statement (§A9.2) or as the left operand

of a comma operator (§A7.18).

An expression may be converted to type void by a cast. For example, a void cast

documents the discarding of the value of a function call used as an expression statement.

void did not appear in the first edition of this book, but has become common since.

A6.8 Pointers to Void

Any pointer to an object may be converted to type void * without loss of informa-tion. If

the result is converted back to the original pointer type, the original pointer is recovered.

Unlike the pointer-to-pointer conversions discussed in §A6.6, which generally require an

explicit cast, pointers may be assigned to and from pointers of type void *• and may be

compared with them.

This interpretation of void * pointers is new; previously, char * pointers played the

role of generic pointer. The ANSI standard specifically blesses the meeting of void *

pointers with object pointers in assignments and relationals, while requiring explicit

casts for other pointer mixtures.

200

REFERENCE MANUAL

APPENDIX A

A 7.

Expressions

The precedence of expression operators is the same a.s the order of the major subsec-

tions of this section, highest precedence first. Thus, for example, the expressions referred to

as the operands of + (§A7.7) are those expressions defined in §§A7.1-A7.6. Within each

subsection, the operators have the same precedence. Left- or right-associativity is specified

in each subsection for the operators discussed therein. The grammar in §A13 incorporates

the precedence and associativity of the operators.

The precedence and associativity of operators is fully specified, but the order of

evaluation of expressions is, with certain exceptions, undefined, even if the subexpressions

involve side effects. That is, unless the definition of an operator guarantees that its operands

are evaluated in a particular order, the implementation is free to evaluate operands in any

order, or even to interleave their evaluation. However, each operator combines the values

produced by its operands in a way compatible with the parsing of the expression in which it

appears.

This rule revokes the previous freedom to reorder expressions with operators

that are mathematically commutative and associative, but can fail to be compu-

tationally associative. The change affects only floating-point computations near

the limits of their accuracy, and situations where overflow is possible.

The handling of overflow, divide check, and other exceptions in expression evaluation is

not defined by the language. Most existing implementations of C ignore overflow in

evaluation of signed integral expressions and assignments, but this behavior is not

guaranteed. Treatment of division by 0, and all floating-point exceptions, varies among

implementations; sometimes it is adjustable by a non-standard library function.

A7. 1 Pointer Generation

If the type of an expression or subexpression is "array of T," for some type T, then the

value of the expression is a pointer to the first object in the array, and the type of the

expression is altered to ''pointer to T." This conversion does not take place if the expression

is the operand of the unary & operator, or of ++, --, sizeof, or as the left operand of an

assignment operator or the . operator. Similarly, an expression of type "function returning

T," except when used as the operand of the & operator, is converted to "pointer to function

returning T."

A7 .2 Primary Expressions

Primary expressions are identifiers, constants, strings, or expressions in parentheses.

primary-expression:

identifier

constant

string

( expression

An identifier is a primary expression, provided it has been suitably declared as dis-

cussed below. Its type is specified by its declaration. An identifier is an lvalue if it refers to

an object (§AS) and if its type is arithmetic, structure, union, or pointer.

A constant is a primary expression. Its type depends on its form as discussed in §A2.5.

A string literal is a primary expression. Its type is originally "array of char" (for wide-

character strings, "array of wchar _ t"), but following the rule given in §A 7 .1, this

SECTION A7 EXPRESSIONS 201

is usually modified to "pointer to char" ( wchar _ t) and the result is a pointer to the

first character in the string. The conversion also does not occur in certain initializers; see

§A8.7.

A parenthesized expression is a primary expression whose type and value are identi-cal

to those of the unadorned expression. The presence of parentheses does not affect whether

the expression is an lvalue.

A7.3 Postfix Expressions

The operators in postfix expressions group left to right.

postfix -expression:

primary-expression

postfix-expression [ expression ]

postfix-expression ( argument-expression-listopt

postfix-expression . identifier

postfix-expression -> identifier

postfix-expression ++

postfix-expression --

argument-expression-list:

assignment -expression

argument-expression-list , assignment-expression

A7.3.1 Array References

A postfix expression followed by an expression in square brackets is a postfix expres-

sion denoting a subscripted array reference. One of the two expressions must have type

"pointer to T', where T is some type, and the other must have integral type; the type of the

subscript expression is T. The expression E 1 [ E2] is identical (by definition) to

* ( ( E 1 ) + ( E2) ) . See §A8.6.2 for further discussion.

A7.3.2 Function Calls

A function call is a postfix expression, called the function designator, followed by

parentheses containing a possibly empty, comma-separated list of assignment expressions

(§A7.17), which constitute the arguments to the function. If the postfix expression con-sists

of an identifier for which no declaration exists in the current scope, the identifier is

implicitly declared as if the declaration

extern int identifier ( ) ;

had been given in the innermost block containing the function call. The postfix expres-sion

(after possible implicit declaration and pointer generation, §A 7.1) must be of type "pointer

to function returning T," for some type T, and the value of the function call has type T.

In the first edition, the type was restricted to "function," and an explicit *

operator was required to call through pointers to functions. The ANSI standard

blesses the practice of some existing compilers by permitting the same syntax

for calls to functions and to functions specified by pointers. The older syntax is

still usable.

The term argument is used for an expression passed by a function call; the term

parameter is used for an input object (or its identifier) received by a function definition,

202 REFERENCE MANUAL APPENDIX A

or described in a function declaration. The terms "actual argument (parameter)" and "formal

argument (parameter)" respectively are sometimes used for the same distinc-tion.

In preparing for the call to a function, a copy is made of each argument; all argument-

passing is strictly by value. A function may change the values of its parame-ter objects,

which are copies of the argument expressions, but these changes cannot affect the values of

the arguments. However, it is possible to pass a pointer on the understanding that the

function may change the value of the object to which the pointer points.

There are two styles in which functions may be declared. In the new style, the types of

parameters are explicit and are part of the type of the function; such a declaration is also

called a function prototype. In the old style, parameter types are not specified. Function

declaration is discussed in §§A8.6.3 and AlO.l.

If the function declaration in scope for a call is old-style, then default argument pro-

motion is applied to each argument as follows: integral promotion (§A6.1) is performed on

each argument of integral type, and each float argument is converted to double. The effect of

the call is undefined if the number of arguments disagrees with the number of parameters in

the definition of the function, or if the type of an argument after promotion disagrees with

that of the corresponding parameter. Type agreement depends on whether the function's

definition is new-style or old-style. If it is old-style, then the comparison is between the

promoted type of the argument of the call, and the promoted type of the parameter; if the

definition is new-style, the promoted type of the argument must be that of the parameter

itself, without promotion.

If the function declaration in scope for a call is new-style, then the arguments are

converted, as if by assignment, to the types of the corresponding parameters of the function's

prototype. The number of arguments must be the same as the number of explicitly described

parameters, unless the declaration's parameter list ends with the ellipsis notation (, ••• ) . In

that case, the number of arguments must equal or exceed the number of parameters; trailing

arguments beyond the explicitly typed parameters suffer default argument promotion as

described in the preceding paragraph. If the definition of the function is old-style, then the

type of each parameter in the prototype visible at the call must agree with the corresponding

parameter in the definition, after the definition parameter's type has undergone argument

promotion.

These rules are especially complicated because they must cater to a mixture of

old- and new-style functions. Mixtures are to be avoided if possible.

The order of evaluation of arguments is unspecified; take note that various compilers

differ. However, the arguments and the function designator are completely evaluated,

including all side effects, before the function is entered. Recursive calls to any function are

permitted.

A7.3.3 Structure References

A postfix expression followed by a dot followed by an identifier is a postfix expres-sion.

The first operand expression must be a structure or a union, and the identifier must name a

member of the structure or union. The value is the named member of the structure or union,

and its type is the type of the member. The expression is an lvalue if the first expression is

an lvalue, and if the type of the second expression is not an array type.

SECTION A7 EXPRESSIONS 103

A postfix expression followed by an arrow (built from - and >) followed by an iden-

tifier is a postfix expression. The first operand expression must be a pointer to a struc-ture

or a union, and the identifier must name a member of the structure or union. The result

refers to the named member of the structure or union to which the pointer expres-sion

points, and the type is the type of the member; the result is an lvalue if the type is not an

array type.

Thus the expression E 1- > MOS is the same as ( * E 1 ) • MOS. Structures and unions are

discussed in §A8.3.

In the first edition of this book, it was already the rule that a member name in such

an expression had to belong to the structure or union mentioned irt the postfix

expression; however, a note admitted that this rule was not firmly enforced. Recent

compilers, and ANSI, do enforce it.

A7.3.4 Postfix lncrementation

A postfix expression followed by a ++ or -- operator is a postfix expression. The value

of the expression is the value of the operand. After the value is noted, the operand is

incremented ( ++) or decremented (--) by I. The operand must be an lvalue; see the

discussion of additive operators (§A7.7) and assignment (§A7.17) for further constraints on

the operand and details of the operation. The result is not an lvalue.

A7.4 Unary Operators

Expressions with unary operators group right-to-left.

unary-expression:

postfix -expression

++ unary-expression

-- unary-expression

unary-operator cast -expression

sizeof unary-expression sizeof (

type-name )

unary-operator: one of

&. * +

A7.4.1 Prefix lncrementation Operators

A unary expression preceded by a ++ or -- operator is a unary expression. The operand

is incremented (++) or decremented (--) by I. The value of the expression is the value after

the incrementation (decrementation). The operand must be an Ivalue; see the discussion of

additive operators (§A7.7) and assignment (§A7.17) for further constraints on the operand

and details of the operation. The result is not an lvalue.

A7.4.2 Address Operator

The unary &. operator takes the address of its operand. The operand must be an lvalue

referring neither to a bit-field nor to an object declared as register, or must be of function

type. The result is a pointer to the object or function referred to by the lvalue. If the type of

the operand is T, the type of the result is "pointer to T."

104 REFERENCE MANUAL APPENDIX A

A7.4.3 Indirection Operator

The unary *operator denotes indirection, and returns the object or function to which its

operand points. It is an lvalue if the operand is a pointer to an object of arithmetic, structure,

union, or pointer type. If the type of the expression is "pointer to T," the type of the result is

A7.4.4 Unary Plus Operator

The operand of the unary + operator must have arithmetic type, and the result is the

value of the operand. An integral operand undergoes integral promotion. The type of the

result is the type of the promoted operand.

The unary + is new with the ANSI standard. It was added for symmetry with unary-.

A7 .4.5 Unary Minus Operator

The operand of the unary - operator must have arithmetic type, and the result is the

negative of its operand. An integral operand undergoes integral promotion. The nega-tive of

an unsigned quantity is computed by subtracting the promoted value from the largest value

of the promoted type and adding one; but negative zero is zero. The type of the result is the

type of the promoted operand.

A7 .4.6 One'sComplement Operator

The operand of the - operator must have integral type, and the result is the one's

complement of its operand. The integral promotions are performed. If the operand is

unsigned, the result is computed by subtracting the value from the largest value of the

promoted type. If the operand is signed, the result is computed by converting the pro-moted

operand to the corresponding unsigned type, applying -, and converting back to the signed

type. The type of the result is the type of the promoted operand.

A7.4.7 Logical Negation Operator

The operand of the I operator must have arithmetic type or be a pointer, and the result is

1 if the value of its operand compares equal to 0, and 0 otherwise. The type of the result is

int.

A7.4.8 Sizeof Operator

The sizeof operator yields the number of bytes required to store an object of the type of

its operand. The operand is either an expression, which is not evaluated, or a parenthesized

type name. When sizeof is applied to a char, the result is 1; when applied to an array, the

result is the total number of bytes in the array. When applied to a structure or union, the

result is the number of bytes in the object, including any padding required to make the

object tile an array: the size of an array of n elements is n times the size of one element. The

operator may not be applied to an operand of func-tion type, or of incomplete type, or to a

bit-field. The result is an unsigned integral con-stant; the particular type is implementation-

defined. The standard header < stddef • h> (see Appendix B) defines this type as size_ t.

SECTION A7 EXPRESSIONS 105

A7.5 Casts

A unary expression preceded by the parenthesized name of a type causes conversion

of the value of the expression to the named type.

cast -expression:

unary-expression

( type-name ) cast-expression

This construction is called a cast.

conversions are described in §A6.

Type names are described in §A8.8. The effects of

An expression with a cast is not an )value.

A7 .6 Multiplicative Operators

The multiplicative operators *• I, and % group left-to-right.

multiplicative-expression:

cast -expression

multiplicative-expression * cast-expression

multiplicative-expression I cast-expression

multiplicative-expression %cast-expression

The operands of * and I must have arithmetic type; the operands of % must have

integral type. The usual arithmetic conversions are performed on the operands, and

predict the type of the result.

The binary * operator denotes multiplication.

The binary I operator yields the quotient, and the % operator the remainder, of the

division of the first operand by the second; if the second operand is 0, the result is unde-

fined. Otherwise, it is always true that (alb) *b + a%b is equal to a. If both operands are

non-negative, then the remainder is non-negative and smaller than the divi-sor; if not, it

is guaranteed only that the absolute value of the remainder is smaller than the absolute

value of the divisor.

A7. 7 Additive Operators

The additive operators + and - group left-to-right.

type, the usual arithmetic conversions are performed.

possibilities for each operator.

If the operands have arithmetic

There are some additional type

additive-expression:

multiplicative-expression

additive-expression + multiplicative-expression

additive-expression - multiplicative-expression

The result of the + operator is the sum of the operands. A pointer to an object in an

array and a value of any integral type may be added. The latter is converted to an

address offset by multiplying it by the size of the object to which the pointer points. The

sum is a pointer of the same type as the original pointer, and points to another object in

the same array, appropriately offset from the original object. Thus if P is a pointer to an

object in an array, the expression P+ 1 is a pointer to the next object in the array. If the

sum pointer points outside the bounds of the array, except at the first loca-tion beyond

the high end, the result is undefined.

The provision for pointers just beyond the end of an array is new. It legitimizes a

common idiom for looping over the elements of an array.

The result of the - operator is the difference of the operands. A value of any

206 REFERENCE MANUAL APPENDIX A

integral type may be subtracted from a pointer, and then the same conversions and con-

ditions as for addition apply.

If two pointers to objects of the same type are subtracted, the result is a signed integral

value representing the displacement between the pointed-to objects; pointers to successive

objects differ by 1. The type of the result depends on the implementation, but is defined as

ptrdiff_ tin the standard header <stddef. h>. The value is undefined unless the pointers point

to objects within the same array; however if P points to the last

member of an array, then ( P+ 1) -P has value

A7.8 Shift Operators

The shift operators « and » group left-to-right. For both operators, each operand

must be integral, and is subject to the integral promotions. The type of the result is that of

the promoted left operand. The result is undefined if 'the right operand is negative, or greater

than or equal to the number of bits in the left expression's type.

shift-expression:

additive-expression

shift-expression < < additive-expression

shift-expression >> additive-expression

The value of E 1 «E2 is E 1 (interpreted as a bit pattern) left-shifted E2 bits; in the absence of

overflow, this is eqqivalent to multiplication by 2E

• The value of E 1 » E2 is E 1 right-

shifted E2 bit positions. The right shift is eq~.Jivalent to division by 2E

if E 1 is unsigned or

if it has a non-negative value; otherwise the result is implementation-<lefined.

A7.9 Relational Operators

The relational operators group left-to-right, but this fact is not useful; a<b<c is parsed as

( a<b) <c, and a<b evaluates to either 0 or 1.

relational-expression:

shift -expression

relational-expression < shift -expression

relational-expression > shift-expression

relational-expression < = shift -expression

relational-expression > = shift -exprfssion

The operators < (less), > (greater), <= Oess or equal) and >= (greater or equal) all yield 0 if

the specified relation is false and 1 if it is true. The type of the result is int. The usual

arithmetic coqversions are performed on arithmetic operands .. Pointers to objects of the

same type (igrioring any qualifiers) may be compared; the result depends on the relative

locations in the address space of the pointed-to objects. Pointer com-parison is defined only

for parts of the same object: if two pointers point to the same simple object, they compare

equal; if the pointers are to members of the same structure, pointers to objects declared later

in the structure compare higher; if the pointers are to members of the same union, they

compare equal; if the pointers refer to members of an array, the comparison is equivalent to

comparison of the corresponding subscripts. If P points to the last member of an array, then

.P+ 1 compares higher than P, even though P+ 1 points outside the array. Otherwise, pointer

comparison is undefined.

These rules slightly liberalize the restrictions stated in the first edition, by per-mitting

comparison of pointers to different members of a structure or union. They also

legalize comparison with a pointer just off the end of an array.

SECTION A7 EXPRESSIONS 207

A 7. 10 Equality Operators

equality-expression:

relational-expression

equality-expression == relational-expression

equality-expression I= relational-expression

The == (equal to) and the I= (not equal to) operators are analogous to the relational operators

except for their lower precedence. (Thus a<b == c<d is I whenever a<b and c<d have the

same truth-value.)

The equality operators follow the

additional possibilities: a pointer may

value 0, or to a pointer to void. See

A7.11 Bitwise AND Operator

same rules as the relational operators, but permit be

compared to a constant integral expression with

§A6.6.

AND-expression:

equality-expression

AND-expression & equality-expression

The usual arithmetic conversions are performed; the result is the bitwise AND function of

the operands. The operator applies only to integral operands.

A7.12 Bitwise Exclusive OR Operator

exclusive-OR -expression:

AND-expression

exclusive-OR-expression " AND-expression

The usual arithmetic conversions are performed; the result is the bitwise exclusive OR

function of the operands. The operator applies only to integral operands.

A7.13 Bitwise Inclusive OR Operator

inclusive-OR -expression:

exclusive-OR -expression

inclusive-OR-expression : exclusive-OR-expression

The usual arithmetic conversions are performed; the result is the bitwise inclusive OR

function of its operands. The operator applies only to integral operands.

A7.14 Logical AND Operator

logical-AND-expression:

inclusive-OR -expression

logical-AND-expression && inclusive-OR-expression

The && operator groups left-to-right. It returns 1 if both its operands compare unequal to

zero, 0 otherwise. Unlike &, && guarantees left-to-right evaluation: the first operand is

evaluated, including all side effects; if it is equal to 0, the value of the expression is 0.

Otherwise, the right operand is evaluated, and if it is equal to 0, the expression's value is 0,

otherwise 1.

The operands need not have the same type, but each must have arithmetic type or be a

pointer. The result is int.

208

REFERENCE MANUAL

APPENDIX A

A7.15

Logical OR Operator

logical- OR- expression:

logical-AND- expression

logical-OR-expression I I

logical-AND-expression

The : : operator groups left-to-right. It returns l if either of its operands compares

unequal to zero, and 0 otherwise. Unlike I, I I guarantees left-to-right evaluation: the

first operand is evaluated, including all side effects; if it is unequal to 0, the value of the

expression is I. Otherwise, the right operand is evaluated, and if it is unequal to 0, the

expression's value is l, otherwise 0.

The operands need not have the same type, but each must have arithmetic type or be a

pointer. The result is int.

A7.16 Qntitional Operator

conditional-expression:

logical- OR- expression

logical-OR-expression? expression : conditional-expression

The first expression is evaluated, including all side effects; if it compares unequal to 0, the

result is the value of the second expression, otherwise that of third expression. Only one of

the second and third operands is evaluated. If the second and third operands are arithmetic,

the usual arithmetic conversions are performed to bring them to a common type, and that is

the type of the result. If both are void, or structures or unions of the same type, or pointers to

objects of the same type, the result has the common type. If one is a pointer and the other the

constant 0, the 0 is converted to the pointer type, and the result has that type. If one is a

pointer to void and the other is another pointer, the other pointer is converted to a pointer to

void, and that is the type of the result.

In the type comparison for pointers, any type qualifiers ( §A8.2) in the type to which the

pointer points are insignificant, but the result type inherits qualifiers from both arms of the

conditional.

A7.17 Assigrmert Expressions

There are several assignment operators; all group right-to-left.

assignment-expression:

conditional- ex pression

unary- ex pression assignment- operator assignment- ex pression

assignment-operator: one of

= *= I= %= ~= -= <<= >>= &= = : =

All require an lvalue as left operand, and the lvalue must be modifiable: it must not be an

array, and must not have an incomplete type, or be a function. Also, its type must not be

qualified with const; if it is a structure or union, it must not have any member or, recursively,

submember qualified with const. The type of an assignment expression is that of its left

operand, and the value is the value stored in the left operand after the assignment has taken

place.

In the simple assignment with =, the value of the expression replaces that of the object

referred to by the lvalue. One of the following must be true: both operands have arithmetic

type, in which case the right operand is converted to the type of the left by the assignment;

or both operands are structures or unions of the same type; or one

SECTION A7 EXPRESSIONS 209

operand is a pointer and the other is a pointer to void; or the left operand is a pointer and the

right operand is a constant expression with value 0; or both operands are pointers to

functions or objects whose types are the same except for the possible absence of const or

volatile in the right operand.

An expression of the form E 1 op = E2 is equivalent to E 1 = E 1 op ( E2 ) except that E1

is evaluated only once.

A7.18 Corrma Operator

expression:

assignment-expression

expression , assignment- expression

A pair of expressions separated by a comma is evaluated left-to-right, and the value of the

left expression is discarded. The type and value of the result are the type and value of the

right operand. All side effects from the evaluation of the left operand are com-pleted before

beginning evaluation of the right operand. In contexts where comma is given a special

meaning, for example in lists of function arguments (§A 7.3 .2) and lists of initializers (

§A8.7), the required syntactic unit is an assignment expression, so the comma operator

appears only in a parenthetical grouping; for example,

f(a, (t=3, t+2), c)

has three arguments, the second of which has the value 5.

A7 .19 Qxlstant Expressions

Syntactically, a constant expression is an expression restricted to a subset of opera-tors:

constant- expression:

conditional- ex pression

Expressions that evaluate to a constant are required in several contexts: after case, as array

bounds and bit-field lengths, as the value of an enumeration constant, in initializ-ers, and in

certain preprocessor expressions.

Constant expressions may not contain assignments, increment or decrement opera-tors,

function calls, or comma operators, except in an operand of sizeof. If the con-stant expression

is required to be integral, its operands must consist of integer, enumera-tion, character, and

floating constants; casts must specify an integral type, and any float-ing constants must be

cast to an integer. This necessarily rules out arrays, indirection, address-of, and structure

member operations. (However, any operand is permitted for

sizeof.)

More latitude is permitted for the constant expressions of initializers; the operands may

be any type of constant, and the unary &. operator may be applied to external or static

objects, and to external or static arrays subscripted with a constant expression. The unary &

operator can also be applied implicitly by appearance of unsubscripted arrays and functions.

Initializers must evaluate either to a constant or to the address of a previously declared

external or static object plus or minus a constant.

Less latitude is allowed for the integral constant expressions after #if; sizeof expressions,

enumeration constants, and casts are not permitted. See §A 12.5.

210 REFERENCE MANUAL APPENDIX A

AS. Declarations

Declarations specify the interpretation given to each identifier; they do not neces· sarily

reserve storage associated with the identifier. Declarations that reserve storage are called

definitions. Declarations have the form

declaration:

declaration-specifiers init -declarator-list

The declarators in the init-declarator-list contain the identifiers being declared; the

declaration-specifiers consist of a sequence of type and storage class specifiers.

declaration-specifiers:

storage-class-specifier dec/aration-specifiersopr

type-specifier declaration-specifiersopt type-

qualifier declaration-specifiersopt

init -declarator-list:

init -declarator

init-declarator-list , init-declarator

init-declarator:

declarator

declarator = initializer

Declarators will be discussed later (§A8.5); they contain the names being declared. A

declaration must have at least one declarator, or its type specifier must declare a struc-ture

tag, a union tag, or the members of an enumeration; empty declarations are not per-mitted.

AS. 1 Storage Class Specifiers

The storage class specifiers are:

storage-class-specifier:

auto

static

extern

typedef

The meanings of the storage classes were discussed in §A4.

The auto and register specifiers give the declared objects automatic storage class, and

may be used only within functions. Such declarations also serve as definitions and cause

storage to be reserved. A register declaration is equivalent to an auto declaration, but hints

that the declared objects will be accessed frequently. Only a few objects are actually placed

into registers, and only certain types are eligible; the restric-tions· are implementation-

dependent. However, if an object is declared register, the unary &. operator may not be

applied to it, explicitly or implicitly.

The rule that it is illegal to calculate the address of an object declared register,

but actually taken to be auto, is new.

The static specifier gives· the declared objects static storage class, and may be used

either inside or outside functions. Inside a function, this specifier causes storage to be

allocated, and serves as a definition; for its effect outside a function, see §All.2.

A declarationwith extern, used inside a function, specifies that the storage for the

declared objects is defined elsewhere; for its effects outside a function, see §All.2.

SECTION A8 DECLARATIONS 111

The typedef specifier does not reserve storage and is called a storage class specifier only

for syntactic convenience; it is discussed in §A8.9.

At most one storage class specifier may be given in a declaration. If none is given, these

rules are used: objects declared inside a function are taken to be auto; functions declared

within a function are taken to be extern; objects and functions declared out-side a function

are taken to be static, with external linkage. See §§AlO-All.

A8.2 Type Specifiers

The type-specifiers are

type-specifier:

void

char

short

int

long

float

double

signed

unsigned

struct -or-union-specifier

enum -specifier

typedef-name

At most one of the words long or short may be specified together with int; the meaning is

the same if int is not mentioned. The word long may be specified together with double. At

most one of signed or unsigned may be specified together with int or any of its short or long

varieties, or with char. Either may appear alone, in which case int is understood. The signed

specifier is useful for forcing char objects to carry a sign; it is permissible but redundant

with other integral types.

Otherwise, at most one type-specifier may be given in a declaration. If the type-specifier

is missing from a declaration, it is taken to be int.

Types may also be qualified, to indicate special properties of the objects being declared.

type-qualifier:

const

volatile

Type qualifiers may appear with any type specifier. A const object may be initialized, but

not thereafter assigned to. There are no implementation-independent semantics for volatile

objects.

The const and volatile properties are new with the ANSI standard. The purpose of

const is to announce objects that may be placed in read-only memory, and perhaps

to increase opportunities for optimization. The purpose of volatile is to force an

implementation to suppress optimization that could otherwise occur. For example,

for a machine with memory-mapped input/output, a pointer to a device register

might be declared as a pointer to volatile, in order to prevent the compiler from

removing apparently redun-dant references through the pointer. Except that it should

diagnose explicit attempts to change canst objects, a compiler may ignore these

qualifiers.

212 REFERENCE MANUAL APPENDIX A

A8.3 Structure and Union Declarations

A structure is an object consisting of a sequence of named members of various

types. A union is an object that contains, at different times, any one of several members

of various types. Structure and union specifiers have the same form.

struct -or-union-specifier:

struct-or-union identifieropr { struct-declaration-list }

struct-or-union identifier

struct -or-union:

struct

union

A struct-declaration-list is a sequence of declarations for the members of the structure

or union:

struct -declaration-list:

struct -declaration

struct -declaration-list struct -declaration

struct -declaration:

specifier-qualifier-list struct -declarator-list

specifier-qualifier-list:

type-specifier specifier-qualifier-/istopt

type-qualifier specifier-qualifier-listopt

struct-declarator-list:

struct-declarator

struct-declarator-list , struct-declarator

Usually, a struct-declarator is just a declarator for a member of a structure or union. A

structure member may also consist of a specified number of bits. Such a member is also

called a bit-field, or merely field; its length is set off from the declarator for the field

name by a colon.

struct -declarator:

declarator

declaratoropt : constant-expression

A type specifier of the form

struct-or-union identifier { struct-declaration-list }

declares the identifier to be the tag of the structure or union specified by the list. A

subsequent declaration in the same or an inner scope may refer to the same type by

using the tag in a specifier without the list:

struct -or-union identifier

If a specifier with a tag but without a list appears when the tag is not declared, an incomplete

type is specified. Objects with an incomplete structure or union type may be mentioned in

contexts where their size is not needed, for example in declarations (not definitions), for

specifying a pointer, or for creating a typedef, but not otherwise. The type becomes complete

on occurrence of a subsequent specifier with that tag, and con-taining a declaration list.

Even in specifiers with a list, the structure or union type being declared is incomplete within

the list, and becomes complete only at the } terminating the specifier.

A structure may not contain a member of incomplete .type. Therefore, it is impossi-

ble to declare a structure or union containing an instance of itself. However, besides

SECTION AB DECLARATIONS 213

giving a name to the structure or union type, tags allow definition of self-referential

structures; a structure or union may contain a pointer to an instance of itself, because

pointers to incomplete types may be declared.

A very special rule applies to declarations of the form

struct-or-union identifier ;

that declare a structure or union, but have no declaration list and no declarators. Even if the

identifier is a structure or union tag already declared in an outer scope (§All.l), this

declaration makes the identifier the tag of a new, incompletely-typed structure or union in

the current scope.

This recondite rule is new with ANSI. It is intended to deal with mutually-

recursive structures declared in an inner scope, but whose tags might already be

declared in the outer scope.

A structure or union specifier with a list but no tag creates a unique type; it can be

referred to directly only in the declaration of which it is a part.

The names of members and tags do not conflict with each other or with ordinary

variables. A member name may not appear twice in the same structure or union, but the

same member name may be used in different structures or unions.

In the first edition of this book, the names of structure and union members were

not associated with their parent. However, this association became common in

compilers well before the ANSI standard.

A non-field member of a structure or union may have any object type. A field member

(which need not have a declarator and thus may be unnamed) has type int, unsigned int, or

signed int, and is interpreted as an object of integral type of the specified length in bits; whether

an int field is treated as signed is implementation-dependent. Adjacent field members of

structures are packed into implementation-dependent storage units in an implementation-

dependent direction. When a field follow-ing another field will not fit into a partially-filled

storage unit, it may be split between units, or the unit may be padded. An unnamed field with

width 0 forces this padding, so that the next field will begin at the edge of the next allocation

unit.

The ANSt standard makes fields even more implementation-dependent than did

the first ~dition. It is advisable to read the language rules for storing bit-fields as

"implementlltion-dependent" without qqalification. Structures with bit-fields

may bC used as a portable way of attempting to reduce the storage required for

a structure (with the probable cost of increasing the instruction space, and time,

needed to access the fields), or as a non-portable way to describe a storage lay-

out known at the bit level. In the second case, it is necessary to understand the

rules of the local implementation.

The members of a structure have addresses increasing in the order of their declara-tions.

A non-field member of a structure is aligned at an addressing boundary depend-ing on its

type; therefore, th(;re may be unnamed holes in a structure. If a pointer to a structure is cast

to the type of a pointer to its first member, the result refers to the first member.

A union may be thought of as a structure all of whose members begin at offset 0 and

whose size is sufficient to contain any of its members. At most one of the members can be

stored in a union at any time. If a pointer to a union is cast to the type of a pointer to a

member, the result refers to that member.

A simple example of a structure declaration is

114

REFERENCE MANUAL

APPENDIX A

struct tnode {

cha:r tword[20];

int count;

struct tnode *left;

struct tnode *right;

} ;

which contains an array of 20 characters, an integer, and two pointers to similar struc-

tures. On~ this declaration has been gi~en, the declaration

struct tnode s, *SP;

declares s to be a structure of the given sort and sp to be a pointer to a structure of the

given sort. With these declarations, the expression

sp->count

refers to the count field of the structure to which sp points;

s.left

refers to the left subtree pointer of the structure s; and

s.right->tword[O)

refers to the first character of the. tword member of the right subtree of s.

In general, a member of a union may not be inspected unless the value of the union

has been assigned using that same member. However, one special guarantee simplifies

the use of unions: if a union contains several structures that share a common initial

s<:quence, and if the union currently contains one of these structures, it is permitted to

refer to the common initial part of any of the contained structures. For example, the

following is a legal fragment:

union {

struct {

int type;

} n;

struct {

int type;

int intnode;

} ni;

struct {

int type;

float floatnode;

} nf;

} u;

u.nf.type = FLOAT;

u.nf.floatnode = 3.14;

if (u.n.type == FLOAT)

... sin(u.nf .floatnode)

A8.4 l:numerationa

Enumerations are unique types with values ranging over a set of named constants

called enumerators. The form of an enumeration specifier borrows from that of struc-

tures and unions.

SECTION AS DECLARATIONS 115

enum-specifier:

enum identifieropt { enumerator-list }

enum identifier

enumerator-list:

enumerator

enumerator-list , enumerator

enumerator:

identifier

identifier = constant-expression

The identifiers in an enumerator list are declared as constants of type int, and may

appear wherever constants are required. If no enumerators with = appear, then the

values of the corresponding constants begin at 0 and increase by 1 as the declaration is

read from left to right. An enumerator with = gives the associated identifier the value

specified; subsequent identifiers continue the progression from the assigned value.

Enumerator names in the same scope must all be distinct from each other and from

ordinary variable names, but the values need not be distinct.

The role of the identifier in the enum-specifier is analogous to that of the structure

tag in a struct-specifier; it names a particular enumeration. The rules for enum-specifiers

with and without tags and lists are the same as those for structure or union specifiers,

except that incomplete enumeration types do not exist; the tag of an enum-specifier

without an enumerator list must refer to an in-scope specifier with a list.

Enumerations are new since the first edition of this book, but have been part of

the language for some years.

A8.5 Declarator&

Declarators have the syntax:

declarator:

pointeropt direct -declarator

direct -declarator:

identifier

( declarator )

direct-declarator [ constant-expressionopt ]

direct-declarator ( parameter-type-list )

direct-declarator ( identifier-listopt )

pointer:

* type-qualifier-listopt

* type-qualifier-listopt pointer

type-qualifier-list:

type-qualifier

type-qualifier-list type-qualifier

The structure of declarators resembles that of indirection, function, and array expres-

sions; the grouping is the same.

216 REFERENCE MANUAL APPENDIX A

A8.6 Meaning of Oeclaratora

A list of declarators appears after a sequence of type and storage class specifiers.

Each declarator declares a unique main identifier, the one that appears as the first alter-

native of the production for direct-declarator. The storage class specifiers apply directly

to this identifier, but its type depends on the form of its declarator. A declarator is read

as an assertion that when its identifier appears in an expression of the same form as the

declarator, it yields an object of the specified type.

Considering only the type parts of the declaration specifiers (§A8.2) and a particular

declarator, a declaration has the form "T D," where T is a type and D is a declarator. The

type attributed to the identifier in the various forms of declarator is described

inductively using this notation.

In a declaration T D where D is an unadorned identifier, the type of the identifier is

In a declaration T D where D has the form

( D1 )

then the type of the identifier in D1 is the same as that of D. The parentheses do not alter

the type, but may change the binding of complex declarators.

A8.6. 1 Pointer Oeclaratora

In a declaration T D where D has the form

* type-qualifier-listopt D 1

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the

identifier of Dis "type-modifier type-qualifier-list pointer to T." Qualifiers follow-ing *

apply to pointer itself, rather than to the object to which the pointer points.

For example, consider the declaration

int *ap[];

Here ap[] plays the role of D1; a declaration "int ap[ ]" (below) would give ap the type

"array of int," the type-qualifier list is empty, and the type-modifier is "array of." Hence

the actual declaration gives ap the type "array of pointers to int."

As other examples, the declarations

int i, *Pi, *COnst cpi = &i;

const int ci = 3, *Pci;

declare an integer i and a pointer to an integer pi. The value of the constant pointer cpi

may not be changed; it will always point to the same location, although the value to which it

refers may be altered. The integer ci is constant, and may not be changed (though it may be

initialized, as here.) The type of pci is "pointer to const int," and pci itself may be changed to

point to another place, but the value to which it points may not be altered by assigning

through pci.

A8.6.2 Array Oeclaratora

In a declaration T D where D has the form

D1 [constant-expressionopt]

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the

identifier of Dis "type-modifier array ofT." If the constant-expression is present, it must

have integral type, and value greater than 0. If the constant expression specifying

SECTION A8 DECLARATIONS 217

the bound is missing, the array has an incomplete type.

An array may be constructed from an arithmetic type, from a pointer, from a struc-

ture or union, or from another array (to generate a multi-dimensional array). Any type

from which an array is constructed must be complete; it must not be an array or struc-

ture of incomplete type. This implies that for a multi-dimensional array, only the first

dimension may be missing. The type of an object of incomplete array type is completed

by another, complete, declaration for the object (§Al0.2), or by initializing it (§A8.7).

For example,

float fa[17], *afp[17];

declares an array of float numbers and an array of pointers to float numbers. Also, static

int x3d[3][5][7];

declares a static three-dimensional array of integers, with rank 3X5x7. In complete

detail, x3d is an array of three items; each item is an array of five arrays; each of the

latter arrays

is an array of seven integers. Any of the

expressions

x3d, x3d [ i ],

x3d [ i] [ j],

x3d [ i] [ j ] [ k] may reasonably appear in an expression.

The first three

have type "array," the last has type int. More specifically, x3d [ i ] [ j

]

is an array of

7 integers, and x3d [ i] is an array of 5 arrays of 7 integers.

The array subscripting operation is defined so that

E 1 [ E2]

identical to

* ( E 1 +E2). Therefore, despite its asymmetric appearance, subscripting is a commuta-

tive operation. Because of the conversion rules that apply to + and to arrays (§§A6.6,

A7.1, A7.7), if E1 is an array and E2 an integer, then E1 [E2] refers to the E2-th

member of E 1.

In the example, x3d[ i] [ j] [k] is equivalent to * ( x3d[ i] [ j] + k ). The first

subexpression x3d [ i] [ j ] is converted by §A 7. I to type "pointer to array of integers;"

by §A7.7, the addition involves multiplication by the size of an integer. It follows from

the rules that arrays are stored by rows Oast subscript varies fastest) and that the first

subscript in the declaration helps determine the amount of storage consumed by an

array, but plays no other part in subscript calculations.

A8.6.3 Function Declarators

In a new-style function declaration T D where D has the form

D 1 (parameter-type-list)

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the

identifier of D is "type-modifier function with arguments parameter-type-list returning T."

The syntax of the parameters is

parameter-type-list:

parameter-list

para111eter-list ,

parameter-list:

parameter-declaration

parameter-list , parameter-declaration

parameter-declaration:

declaration-specifiers declarator

declaration-specifiers abstract-declaratoropt

In the new-style declaration, the parameter list specifies the types of the parameters. As

118 REFERENCE MANUAL APPENDIX A

a special case, the declarator for a new-style function with no parameters has a parame-

ter type list consisting solely of the keyword void. If the parameter type list ends with an

ellipsis ", ••• ", then the function may accept more arguments than the number of

parameters explicitly described; see §A 7.3 .2.

The types of parameters that are arrays or functions are altered to pointers, in

accordance with the rules for parameter conversions; see §AlO.l. The only storage class

specifier permitted in a parameter's declaration specifier is register, and this specifier is

ignored unless the function declarator heads a function definition. Similarly, if the

declarators in the parameter declarations contain identifiers and the function declarator

does not head a function definition, the identifiers go out of scope immediately. Abstract

declarators, which do not mention the identifiers, are discussed in §A8.8.

In an old-style function declaration T D where D has the form D

1 (identifier-list opt)

and the type of the identifier in the declaration T D1 is "type-modifier T," the type of the

identifier of D is "type-modifier function of unspecified arguments returning T." The

parameters (if present) have the form

identifier-list:

identifier

identifier-list , identifier

In the old-style declarator, the identifier list must be absent unless the declarator is used

in the head of a function definition (§AlO.l). No information about the types of the

parameters is supplied by the declaration.

For example, the declaration

int f(), *fpi(), (*pfi)();

declares a function f returning an integer, a function fpi returning a pointer to an integer,

and a pointer pf i to a function returning an integer. In none of these are the parameter

types specified; they are old-style.

In the new-style declaration

int strcpy(char *dest, const char *Source), rand(void); strcpy is a function

returning int, with two arguments, the first a character pointer, and the second a pointer

to constant characters. The parameter names are effectively comments. The second

function rand takes no arguments and returns int.

Function declarators with parameter prototypes are, by far, the most important

language change introduced by the ANSI standard. They offer an advantage over

the "old-style" declarators of the first edition by providing error-detection and

coercion of arguments across function calls, but at a cost: turmoil and con-fusion

during their introduction, and the necessity of accommodating both forms. Some

syntactic ugliness was required for the sake of compatibility, namely void as an

explicit marker of new-style functions without parameters.

The ellipsis notation ", •.• " for variadic functions is also new, and, together

with the macros in the standard header <stdarg. h>, formalizes a mechanism that

was officially forbidden but unofficially condoned in the first edition.

These notations were adapted from the C++ language.

AS. 7 Initialization

When an object is declared, its init-declarator may specify an initial value for the

identifier being declared. The initializer is preceded by =, and is either an expression, or

a list of initializers nested in braces. A list may end with a comma, a nicety for neat

SECTION AS DECLARATIONS 219

formatting.

initializer:

assignment -expression

{ initia/izer-list }

{ initializer-list , }

initializer-list:

initializer

initializer-list , initializer

All the expressions in the initializer for a static object or array must be constant

expressions as described in §A7.19. The expressions in the initializer for an auto or

enclosed list. However, if the initializer for an automatic object is a single expres-sion, it

need not be a constant expression, but must merely have appropriate type for assignment to

the object.

The first edition did not countenance initialization of automatic structures,

unions, or arrays. The ANSI standard allows it, but only by constant construc-

tions unless the initializer can be expressed by a simple expression.

A static object not explicitly initialized is initialized as if it (or its members) were

assigned the constant 0. The initial value of an automatic object not explicitly initialized

is undefined.

The initializer for a pointer or an object of arithmetic type is a single expression,

perhaps in braces. The expression is assigned to the object.

The initializer for a structure is either an expression of the same type, or a brace-

enclosed list of initializers for its members in order. Unnamed bit-field members are

ignored, and are not initialized. If there are fewer initializers in the list than members of

the structure, the trailing members are initialized with 0. There may not be more ini-

tializers than members.

The initializer for an array is a brace-enclosed list of initializers for its members. If

the array has unknown size, the number of initializers determines the size of the array,

and its type becomes complete. If the array has fixed size, the number of initializers may

not exceed the number of members of the array; if there are fewer, the trailing members

are initialized with 0.

As a special case, a character array may be initialized by a string literal; successive

characters of the string initialize successive members of the array. Similarly, a wide

character literal (§A2.6) may initialize an array of type wchar _ t. If the array has

unknown size, the number of characters in the string, including the terminating null

character, determines its size; if its size is fixed, the number of characters in the string,

not counting the terminating null character, must not exceed the size of the array.

The initializer for a union is either a single expression of the same type, or a brace-

enclosed initializer for the first member of the union.

The first edition did not allow initialization of unions. The "first-member" rule is

clumsy, but is hard to generalize without new syntax. Besides allowing unions to

be explicitly initialized in at least a primitive way, this ANSI rule makes definite

the semantics of static unions not explicitly initialized.

An aggregate is a structure or array. If an aggregate contains members of aggregate

type, the initialization rules apply recursively. Braces may be elided in the initialization

as follows: if the initializer for an aggregate's member that is itself an aggregate begins

with a left brace, then the succeeding comma-separated list of initializers initializes the

110 REFERENCE MANUAL APPENDIX A

members of the subaggregate; it is erroneous for there to be more initializers than

members. If, however, the initializer for a subaggregate does not begin with a left brace,

then only enough elements from the list are taken to account for the members of the

subaggregate; any remaining members are left to initialize the next member of the

aggregate of which the subaggregate is a part.

For example,

int x[] = { 1, 3, 5 };

declares and initializes xas a !-dimensional array with three members, since no size was

specified and there are three initializers.

float y[4][3] = {

{ 1, 3, 5 },

{ 2, 4, 6 }, { 3, 5,

7 },

} ;

is a completely-bracketed initialization: 1, 3, and 5 initialize the first row of the array

y[O], namely y[O][O], y[0][1], and y[0][2]. Likewise the next two lines initial-ize y [ 1 ] and y [

2]. The initializer ends early, and therefore the elements of y [ 3] are initialized with 0.

Precisely the same effect could have been achieved by

float y[4][3] = {

1, 3, 5, 2, 4, 6, 3, 5, 7

} ;

The initializer for y begins with a left brace, but that for y [ 0] does not; therefore three

elements from the list are used. Likewise the next three are taken successively for y [ 1 ]

and then for y [ 2 ] . Also,

float y[4][3] = {

{ 1 }, { 2 }, { 3 }, { 4 }

} ;

initializes the first column of y (regarded as a two-dimensional array) and leaves the rest

Finally,

char msq[] = "Syntax error on line "s\n";

shows a character array whose members are initialized with a string; its size includes the

terminating null character.

A8.8 Type Namea

In several contexts (to specify type conversions explicitly with a cast, to declare

parameter types in function declarators, and as an argument of sizeof) it is necessary to

supply the name of a data type. This is accomplished using a type name, which is

syntactically a declaration for an object of that type omitting the name of the object.

type-name:

specifier-qualifier-list abstract -declaratoropt

abstract -declarator:

pointer

pointeropt direct -abstract -declarator

SECTION AS DECLARATIONS lll

direct -abstract -declarator:

( abstract-declarator )

direct -abstract -declaratoropt [ constant -expressionopt ]

direct-abstract-declaratoropt parameter-type-list

)

It is possible to identify uniquely the location in the abstract-declarator where the identi-fier

would appear if the construction were a declarator in a declaration. The named type is then

the same as the type of the hypothetical identifier. For example,

int

int *

int *[3]

int ( *) []

int *()

int (*[])(void)

name respectively the types "integer," "pointer to integer," "array of 3 pointers to integers,"

"pointer to an array of an unspecified number of integers," "function of unspecified

parameters returning pointer to integer," and "array, of unspecified size, of pointers to

functions with no parameters each returning an integer."

A8.9 Typedef

Declarations whose storage class specifier is typedef do not declare objects; instead they

define identifiers that name types. These identifiers are called typedef names.

typedef-name:

identifier

A typedef declaration attributes a type to each name among its declarators in the usual way

(see §8.6). Thereafter, each such typedef name is syntactically equivalent to a type specifier

keyword for the associated type.

For example, after

typedef lonq Blockno, *Blockptr;

typedef struct { double r, theta; } Complex;

the constructions

Blockno b;

extern Blockptr bp;

Complex z, *ZP;

are legal declarations. The type of b is lonq, that of bp is "pointer to lonq," and that of z is

the specified structure; zp is a pointer to such a structure.

typedef does not introduce new types, only synonyms for types that could be speci-fied

in another way. In the example, b has the same type as any other lonq object.

Typedef names may be redeclared in an inner scope, but a non-empty set of type

specifiers must be given. For example,

extern Blockno;

does not redeclare Blockno, but

extern int Blockno;

does.

A8.10 Type Equivalence

Two type specifier lists are equivalent if they contain the same set of type specifiers,

taking into account that some specifiers can be implied by others (for example, lonq

222 REFERENCE MANUAL APPENDIX A

alone implies long int). Structures, unions, and enumerations with different tags are

distinct, and a tagless union, structure, or enumeration specifies a unique type.

Two types are the same if their abstract declarators (§A8.8), after expanding any

typedef types, and deleting any function parameter identifiers, are the same up to

equivalence of type specifier lists. Array sizes and function parameter types are signifi-cant.

A9. Statements

Except as described, statements are

for their effect, and do not have values.

executed in sequence. Statements are executed

They fall into several groups.

statement:

labeled -statement

expression-statement

compound -statement

selection-statement

iteration-statement

jump-statement

A9.1 Labeled Statements

Statements may carry label prefixes.

labeled -statement:

identij1er : statement

case constant-expression : statement

default : statement

A label consisting of an identifier declares the identifier. The only use of an identifier

label is as a target of goto. The scope of the identifier is the current function. Because

labels have their own name space, they do not interfere with other identifiers and cannot

be redeclared. See §A 11.1.

Case labels and default labels are used with the switch statement (§A9.4). The

constant expression of case must have integral type.

Labels in themselves do not alter the flow of control.

A9.2 Expression Statement

Most statements are expression statements, which have the form

expression-statement:

expressionopt ;

Most expression statements are assignments or function calls. All side effects from the

expression are completed before the next statement is executed. If the expression is

missing, the construction is called a null statement; it is often used to supply an empty

body to an iteration statement or to place a label.

A9.3 Compound Statement

So that several statements can be used where one is expected, the compound state-

ment (also called "block") is provided. The body of a function definition is a compound

statement.

SECTION A9 STATEMENTS 223

compound -statement:

{ declaration-/istopt statement-listopt }

declaration-list:

declaration

declaration-list declaration

statement-list:

statement

statement-list statement

If an identifier in the declaration-list was in scope outside the block, the outer declara· tion is

suspended within the block (see §All.l), after which it resumes its force. An identifier may

be declared only once in the same block. These rules apply to identifiers in the same name

space (§All); identifiers in different name spaces are treated as dis-tinct.

Initialization of automatic objects is performed each time the block is entered at the

top, and proceeds in the order of the declarators. If a jump into the block is executed,

these initialization& are not performed. Initializations of static objects are performed

only once, before the program begins execution.

A9.4 Selection Statements

Selection statements choose one of several flows of control.

selection-statement:

if ( expression ) statement

if ( expression ) statement else statement

switch ( expression ) statement

In both forms of the if statement, the expression, which must have arithmetic or

pointer type, is evaluated, including all side-effects, and if it compares unequal to 0, the

first substatement is executed. In the second form, the second substatement is executed

if the expression is 0. The else ambiguity is resolved by connecting an else with the last

encountered else-less if at the same block nesting level.

The switch statement causes control to be transferred to one of several statements

depending on the value of an expression, which must have integral type. The substate·

ment controlled by a switch is typically compound. Any statement within the sub-

statement may be labeled with one or more case labels (§A9.1). The controlling

expression undergoes integral promotion (§A6.1), and the case constants are converted

to the promoted type. No two of the case constants associated with the same switch may

have the same value after conversion. There may also be at most one default label

associated with a switch. Switches may be nested; a case or default label is associ-ated

with the smallest switch that contains it.

When the switch statement is executed, its expression is evaluated, including all side

effects, and compared with each case constant. If one of the case constants is equal to

the value of the expression, control passes to the statement of the matched case label. If

no case constant matches the expression, and if there is a default label, con-trol passes

to the labeled statement. If no case matches, and if there is no default, then none of the

substatements of the switch is executed.

In the first edition of this book, the controlling expression of switch, and the case

constants, were required to have int type.

124

REFERENCE MANUAL

APPENDIX A

A9.5

Iteration Statements

Iteration statements specify looping.

iteration-statement:

while ( expression ) statement

) ;

do statement while

( expression

for ( expressionopr ;

expressionopr

; expressionopr ) statement

In the while and do statements, the substatement is executed repeatedly so long as

the value of the expression remains unequal to 0; the expression must have arithmetic or

pointer type. With while, the test, including all side effects from the expression, occurs

before each execution of the statement; with do, the test follows each iteration.

In the for statement, the first expression is evaluated once, and thus specifies ini-

tialization for the loop. There is no restriction on its type. The second expression must

have arithmetic or pointer type; it is evaluated before each iteration, and if it becomes

equal to 0, the for is terminated. The third expression is evaluated after each iteration,

and thus specifies a re-initialization for the loop. There is no restriction on its type. Side-

effects from each expression are completed immediately after its evaluation. If the

substatement does not contain continue, a statement

for ( expression} ; expression2 ; expression3 ) statement

is equivalent to

expression} ;

while ( expression2 ) {

statement

expression3 ;

}

Any of the three expressions may be dropped. A missing second expression makes

the implied test equivalent to testing a non-zero constant.

A9.6 Jump Statements

Jump statements transfer control unconditionally.

jump-statement:

goto identifier ;

continue ;

break ;

return expressionopr

In the goto statement, the identifier must be a label (§A9.l) located in the current

function. Control transfers to the labeled statement.

A continue statement may appear only within an iteration statement. It causes control

to pass to the loop-continuation portion of the smallest enclosing such statement. More

precisely, within each of the statements

while ( ... ) {

do {

for ( ... ) {

contin: ;

contin:

...

)

;

contin: ;

}

} while (

}

a continue not contained in a smaller iteration statement is the same as goto contin.

A break statement may appear only in an iteration statement or a switch state-ment,

and terminates execution of the smallest enclosing such statement; control passes

SECTION AlO EXTERNAL DECLARATIONS 225

to the statement following the terminated statement.

A function returns to its caller ·by the return statement. When return is followed by an

expression, the value is returned to the caller of the function. The expression is converted, as

if by assignment, to the type returned by the function in which it appears.

Flowing off the end of a function is equivalent to a return with no expression. In either

case, the returned value is undefined.

A 10. External Declarations

The unit of input provided to the C compiler is called a translation unit; it consists of a

sequence of external declarations, which are either declarations or function definitions.

translation-unit:

external-declaration

translation-unit external-declaration

external-declaration:

function-definition

declaration

The scope of external declarations persists to the end of the translation unit in which

they are declared, just as the effect of declarations within blocks persists to the end of the

block. The syntax of external declarations is the same as that of all declarations, except that

only at this level may the code for functions be given.

A 10. 1 Function Definitions

Function definitions have the form

function-definition:

declaration-specifiers

declarator dec/aration-listopt compound-statement

The only storage-class specifiers allowed among the declaration specifiers are extern or

static; see §All.2 for the distinction between them.

A function may return an arithmetic type, a structure, a union, a pointer, or void, but not

a function or an array. The declarator in a function declaration must specify explicitly that

the declared identifier has function type; that is, it must contain one of the forms (see

§A8.6.3)

direct-declarator ( parameter-type-list )

direct-declarator ( identifier-listopt )

where the direct-declarator is an identifier or a parenthesized identifier. In particular, it must

not achieve function type by means of a typedef.

In the first form, the definition is a new-style function, and its parameters, together with

their types, are declared in its parameter type list; the declaration-list following the

function's declarator must be absent. Unless the parameter type list consists solely of void,

showing that the function takes no parameters, each declarator in the parameter

type list must contain an identifier. If the parameter type list ends with "

••• " then the

function may be called with more arguments than parameters; the va_arg macro mechanism

defined in the standard header <stdarg. h> and described in Appendix B must be used to

refer to the extra arguments. Variadic functions must have at least one named parameter.

In the second form, the definition is old-style: the identifier list names the

226 REFERENCE MANUAL APPENDIX A

parameters, while the declaration list attributes types to them. If no declaration is given

for a parameter, its type is taken to be int. The declaration list must declare only

parameters named in the list, initialization is not permitted, and the only storage-class

specifier possible is register.

In both styles of function definition, the parameters are understood to be declared

just after the beginning of the compound statement constituting the function's body, and

thus the same identifiers must not be redeclared there (although they may, like other

identifiers, be redeclared in inner blocks). If a parameter is declared to have type "array

of type," the declaration is adjusted to read "pointer to type;" similarly, if a parameter is

declared to have type "function returning type," the declaration is adjusted to read

"pointer to function returning type." During the call to a function, the argu-ments are

converted as necessary and assigned to the parameters; see §A 7 .3.2.

New-style function definitions are new with the ANSI standard. There is also a

small change in the details of promotion; the first edition specified that the

declarations of float parameters were adjusted to read double. The differ-ence

becomes noticeable when a pointer to a parameter is generated within a function.

A complete example of a new-style function definition is

int max(int a, int b, int c)

{

int m;

m = (a > b) ? a : b;

return (m > c) ? m : c;

}

Here int is the declaration specifier; max ( int a, int b, int c) is the function's

declarator, and { o o o } is the block giving the code for the function. The correspond-ing old-

style definition would be

int max(a,

int a, b,

{

}

where now int max (a, b,

c) is the declarator, and int

is the declara-

tion list for the parameters.

A 10.2 External Declarations

External declarations specify the characteristics of objects, functions and other iden-

tifiers. The term "external" refers to their location outside functions, and is not directly

connected with the extern keyword; the storage class for an externally-declared object

may be left empty, or it may be specified as extern or static.

Several external declarations for the same identifier may exist within the same trans-

lation unit if they agree in type and linkage, and if there is at most one definition for the

identifier.

Two declarations for an object or function are deemed to agree in type under the

rules discussed in §A8.10. In addition, if the declarations differ because one type is an

incomplete structure, union, or enumeration type (§A8.3) and the other is the

corresponding completed type with the same tag, the types are taken to agree. More-

over, if one type is an incomplete array type (§A8.6.2) and the other is a completed

SECTION All SCOPE AND LINKAGE 227

array type, the types, if otherwise identical, are also taken to agree. Finally, if one type

specifies an old-style function, and the other an otherwise identical new-style function, with

parameter declarations, the types are taken to agree.

If the first external declaration for a function or object includes the static speci-fier, the

identifier has internal linkage; otherwise it has external linkage. Linkage is discussed in

§All.2.

An external declaration for an object is a definition if it has an initializer. An exter-nal

object declaration that does not have an initializer, and does not contain the extern specifier,

is a tentative definition. If a definition for an object appe~J,rs in a ttanslation unit, any

tentative definitions are treated merely as redundant declarations. If no defini-tion for the

opject appears in the translation unit, all its tentative definitions become a single definition

with initializer 0.

Each object must have exactly one definition. For objects with internal linkage, this rule

applies separately to each translation unit, because internally-linked objects are unique to a

translation unit. For objects with external linkage, it applies to the entire program.

Although the one-definition rule is formulated somewhat differently in the first

edition of this book, it is in· effect identical to the one 'Stated here. Some imple-

mentations relax it by generalizing the notion of tentative definition. In the

alternate formulation, which is usual in UNIX systems and recognized as a com-

mon extension by the Standard, all the tentative definitions for an externally-

linked object, throughout all the translation units of a program, are considered

together instead of in each· translation unit separately. If a definition occurs

somewhere in the program, then the tentative definitions become merely

declarations, but if no definition appears, then all its tentative definitions become

a definition with initializer 0.

A 11. Scope and Linkage

A program need not all be compiled at one time: the source text may be kept in several

files containing translation units, and precompiled routines may be loaded from libraries.

Communication among the functions of a program may be carried out both through calls

and through manipulation of external data.

Therefore, there are two kinds of scope to consider: first, the lexical scope of an

identifier, which is the region of the program text within which the identifier's charac-

teristics are understood; and seci:>nd, the scope associated with objects and functions with

external linkage, which determines the connections between identifiers in separately com-

piled translation units.

A 11. 1 Lexical Scope

Identifiers fall into several name spaces that do not interfere with one another; the same

identifier may be used for different purposes, even in the same scope, if the uses are in

different name spaces. These classes are: objects, functions, typedef names, and enum

constants; labels; tags of structures, unions, and enumerations; and members of each

structure or union individually.

These rules differ in several ways from those described in the first edition of this

manual. Labels did not previo\lsly have their own name space; tags of structures

and unions each had a separate space, and in some implementations

128 REFERENCE MANUAL APPENDIX A

enumeration tags did as well; putting different kinds of tags into the same space

is a new restriction. The most important departure from the first edition is that

each structure or union creates a separate name space for its members, so that the

same name may appear in several different structures. This rule has been

common practice for several years.

The lexical scope of an object or function identifier in an external declaration begins

at the end of its declarator and persists to the end of the translation unit in which it

appears. The scope of a parameter of a function definition begins at the start of the

block defining the function, and persists through the function; the scope of a parameter

in a function declaration ends at the end of the declarator. The scope of an identifier

declared at the head of a block begins at the end of its declarator, and persists to the end

of the block. The scope of a label is the whole of the function in which· it appears. The

scope of a structure, union, or enumeration tag, or an enumeration constant, begins at its

appearance in a type specifier, and persists to the end of the translation unit (for

declarations at the external level) or to the end of the block (for declarations within a

function).

If an identifier is explicitly declared at the head of a block, including the block con-

stituting a function, any declaration of the identifier outside the block is suspended until the

end of the block.

A 11.2 Linkage

Within a translation unit, all declarations of the same object or function identifier

with internal linkage refer to the same thing, and the object or function is unique to that

translation unit. All declarations for the same object or function identifier with external

linkage refer to the same thing, and the object or function is shared by the entire pro-

gram.

As discussed in §A10.2, the first external declaration for an identifier gives the iden-

tifier internal linkage if the static specifier is used, external linkage otherwise. If a

declaration for an identifier within a block does not include the extern specifier, then the

identifier has no linkage and is unique to the function. If it does include extern, and an

external declaration for the identifier is active in the scope surrounding the block, then

the identifier has the same linkage as the external declaration, and refers to the same

object or function; but if no external declaration is visible, its linkage is external.

A 12. Preprocessing

A preprocessor performs macro substitution, conditional compilation, and inclusion

of named files. Lines beginning with #, perhaps preceded by white space, communicate

with this preprocessor. The syntax of these lines is independent of the rest of the

language; they may appear anywhere and have effect that lasts (independent of scope)

until the end of the translation unit. Line boundaries are significant; each line is

analyzed individually (but see §Al2.2 for how to adjoin lines). To the preprocessor, a

token is any language token, or a character sequence giving a file name as in the #include

directive (§A12.4); in addition, any character not otherwise defined is taken as a token.

However, the effect of white space characters other than space and horizon-tal tab is

undefined within preprocessor lines.

Preprocessing itself takes place in several logically successive phases that may, in a

SECTION Al2 PREPROCESSING 229

particular implementation, be condensed.

1. First, trigraph sequences as described in §A 12.1 are replaced by their equivalents. Should

the operating system environment require it, newline characters are intro-duced between the

lines of the source file.

2. Each occurrence of a backslash character \ followed by a newline is deleted, thus splicing

lines (§A12.2).

3. The program is split into tokens separated by white-space characters; comments are replaced

by a single space. Then preprocessing directives are obeyed, and macros (§§A12.3-Al2.10)

are expanded.

4. Escape sequences in character constants and string literals (§§A2.5.2, A2.6) are replaced by

their equivalents; then adjacent string literals are concatenated.

5. The result is translated, then linked together with other programs and libraries, by collecting

the necessary programs and data, and connecting external function and object references to

their definitions.

A 12. 1 Trigraph Sequences

The character set of C source programs is contained within seven-bit ASCII, but is a superset

of the ISO 646-1983 Invariant Code Set. In order to enable programs to be represented in the

reduced set, all occurrences of the following trigraph sequences are replaced by the

corresponding single character. This replacement occurs before any other processing.

??=

??(

??<

{

??/

??)

??>

}

??'

??I

??-

No other such replacements occur.

Trigraph sequences are new with the ANSI standard.

A 12.2 Line Splicing

Lines that end with the backslash character \ are folded by deleting the backslash and the

following newline character. This occurs before division into tokens.

A 12.3 Macro 1\)efinition and Expansion

A control line of the form

# define identifier token-sequence

causes the preprocessor to replace subsequent instances of the identifier with the given sequence

of tokens; leading and trailing white space around the token sequence is dis-carded. A second

#define for the same identifier is erroneous unless the second token sequence is identical to the

first, where all white space separations are taken to be equivalent.

A line of the form

# de£ ine identifier ( identifier-list ) token-sequence

where there is no space between the first identifier and the ( , is a macro definition with

parameters given by the identifier list. As with the first form, leading and trailing white space

around the token sequence is discarded, and the macro may be redefined only with

230 REFERENCE MANUAL APPENDIX A

a definition in which the number and spelling of parameters, and the token sequence, is

identical.

A control line of the form

# undef identifier

causes the identifier's preprocessor definition to be forgotten. It is not erroneous to apply

#undef to an unknown identifier.

When a macro has been defined in the second form, subsequent textual instances of

the macro identifier followed by optional white space, and then by (, a sequence of

tokens separated by commas, and a ) constitute a call of the macro. The arguments of the

call are the comma-separated token sequences; commas that are quoted or protected by

nested parentheses do not separate arguments. During collection, arguments are no~

macro-expanded. The number of arguments in the call must match the number of

parameters in the definition. After the arguments are isolated, leading and trailing white

space is removed from them. Then the token sequence resulting from each argu-ment is

substituted for each unquoted occurrence of the corresponding parameter's iden-tifier in

the replacement token sequence of the macro. Unless the parameter in the replacement

sequence is preceded by #, or preceded or followed by ##, the argument tokens are

examined for macro calls, and expanded as necessary, just before insertion.

Two special operators influence the replacement process. First, if an occurrence of a

parameter in the replacement token sequence is immediately preceded by #, string

quotes ( ") are placed around the corresponding parameter, and then both the # and the

parameter identifier are replaced by the quoted argument. A \ character is inserted

before each " or \character that appears surrounding, or inside, a string literal or char-

acter constant in the argument.

Second, if the definition token sequence for either kind of macro contains a ##

operator, then just after replacement of the parameters, each ##is deleted, together with

any white space on either side, so as to concatenate the adjacent tokens and form a new

token. The effect is undefined if invalid tokens are produced, or if the result depends on

the order of processing of the ##operators. Also, ##may not appear at the beginning or

end of a replacement token sequence.

In both kinds of macro, the replacement token sequence is repeatedly rescanned for

more defined identifiers. However, once a given identifier has been replaced in a given

expansion, it is not replaced if it turns up again during rescanning; instead it is left

unchanged.

Even if the final value of a macro expansion begins with #, it is not taken to be a

preprocessing directive.

The details of the macro-expansion process are described more precisely in the

ANSI standard than in the first edition. The most important change is the addi-tion of

the # and ## operators, which make quotation and concatenation admis-sible. Some

of the new rules, especially those involving concatenation, are bizarre. (See example

below.)

For example, this facility may be used for "manifest constants," as

in #define TABSIZE 100

int tabletTABSIZE];

The definition

#define ABSDIFF(a, b) ((a)>(b) ? (a)-(b) : (b)-(a))

defines a macro to return the absolute value of the difference between its arguments.

Unlike a function to do the same thing, the arguments and returned value may have any

SECTION Al2 PREPROCESSING 231

arithmetic type or even be pointers. Also, the arguments, which might have side effects,

are evaluated twice, once for the test and once to produce the value.

Given the definition

#define tempfile(dir) #dir "/%s"

the macro call tempfile(/usr/tmp) yields

"/usr/tmp" "/%s"

which will subsequently be catenated into a single string. After

#define cat(x, y) x ## y

the call cat ( var, 123) yields var 123. However, the call cat (cat ( 1, 2) , 3) ) is

undefined: the presence of ## prevents the arguments of the outer call from being

expanded. Thus it produces the token string

cat ( 1

2 ) 3

and ) 3 (the catenation of the last token of the first argument with the first token of the

second) is not a legal token. If a second level of macro definition is introduced,

#define xcat(x,y) cat(x,y)

things work more smoothly; xcat ( xcat ( 1 , 2 ) , 3 ) does produce 12 3, because the

expansion of xcat itself does not involve the ##operator.

Likewise, ABSDIFF ( ABSDIFF (a, b) , c) produces the expected, fully-expanded result.

A 12.4 File Inclusion

A control line of the form

# include <filename>

causes the replacement of that line by the entire contents of the file filename. The

characters in the name filename must not include > or newline, and the effect is unde-

fined if it contains any of ", ', \, or I*· The named file is searched for in a sequence of

implementation-dependent places.

Similarly, a control line of the form

# include ''filename"

searches first in association with the original source file (a deliberately implementation-

dependent phrase), and if that search fails, then as if in the first form. The effect of using

', \, or I* in the filename remains undefined, but > is permitted.

Finally, a directive of the form

# include token-sequence

not matching one of the previous forms is interpreted by expanding the token sequence

as for normal text; one of the two forms with <... > or " ... " must result, and it is then

treated as previously described.

#include files may be nested.

A 12.5 Conditional Compilation

Parts of a program may be compiled conditionally, according to the following

schematic syntax.

131 REFERENCE MANUAL APPENDIX A

preprocessor-conditional:

if-line text elif-parts e/se-partopt #endif

if-line:

# if constant-expression

# ifdef identifier

# ifndef identifier

elif-parts:

elif-line text

elif-partsopt

elif-line:

# elif constant-expression

else-part:

else-line text

else-line:

# else

Each of the directives (if-line, elif-line, else-line, and #endif) appears alone on a line. The

constant expressions in #if and subsequent #elif lines are evaluated in order until an

expression with a non-zero value is found; text following a line with a zero value is

discarded. The text following the successful directive line is treated normally. "Text" here

refers to any material, including preprocessor lines, that is not part of the condi-tional

structure; it may be empty. Once a successful #if or #elif line has been found and its text

processed, succeeding #elif and #else lines, together with their text, are discarded. If all the

expressions are zero, and there is an #else, the text following the #else is treated normally.

Text controlled by inactive arms of the conditional is ignored except for checking the

nesting of conditionals. .

The constant expression in #if and #elif is subject to ordinary macro replace-ment.

Moreover, any expressions of the form

defined identifier

defined ( identifier )

are replaced, before scanning for macros, by 1L if the identifier is defined in the prepro-

cessor, and by OL if not. Any identifiers remaining after macro expansion are replaced by

OL. Finally, each integer constant is considered to be suffixed with L, so that all arithmetic is

taken to be long or unsigned long.

The resulting constant expression (§A7.19) is restricted: it must be integral, and may not

contain sizeof, a cast, or an enumeration constant.

The control lines

#ifdef identifier

#ifndef identifier

are equivalent to

I if defined identifier

I if I defined identifier

respectively.

#elif is new since the first edition, although it has been available in some

preprocessors. The defined preprocessor operator is also new.

SECTION A12 PREPROCESSING 233

A 12.6 Line Control

For the benefit of other preprocessors that generate C programs, a line in one of the

forms

I line constant "filename" I

line constant

causes the compiler to believe, for purposes of error diagnostics, that the line number of the

next source line is given by the decimal integer constant and the current input file is named

by the identifier. If the quoted filename is absent, the remembered name does not change.

Macros in the line are expanded before it is interpreted.

A 12.7 Error Generation

A preprocessor line of the form

I error token-sequenceopr

causes the processor to write a diagnostic message that includes the token sequence.

A 12.8 Pragmas

A control line of the form

I pragma token-sequenceopr

causes the processor to perform an implementation-dependent action. An unrecognized

pragma is ignored.

A 12.9 Null Directive

A preprocessor line of the form

has no effect.

A12.10 Predefined Names

Several identifiers are predefined, and expand to produce special information. They, and

also the preprocessor expression operator defined, may not be undefined or rede-fined.

LINE

FILE

DATE

TIME __

STDC __

A decimal constant containing the current source line number.

A string literal containing the name of the file being compiled.

A string literal containing the date of compilation, in the form "Mmm dd

yyyy".

A string literal containing the time of compilation, in the form

"hh:mm:ss".

The constant 1. It is intended that this identifier be defined to be 1 only in

standard-conforming implementations.

#error and #pragma are new with the ANSI standard; the

predefined preprocessor macros are new, but some of them have

been available in some implementations.

134 REFERENCE MANUAL APPENDIX A

A 13. Grammar

Below is a recapitulation of the grammar that was given throughout the earlier part

of this appendix. It has exactly the same content, but is in a different order.

The grammar has undefined terminal symbols integer-constant, character-constant,

floating-constant, identifier, string, and enumeration-constant; the typewriter style words

and symbols are terminals given literally. This grammar can be transformed mechanically

into input acceptable to an automatic parser-generator. Besides adding whatever syntactic

marking is used to indicate alternatives in productions, it is necessary to expand the "one or•

constructions, and (depending on the rules of the parser-generator) to duplicate each

production with an opt symbol, once with the symbol and once without. With one further

change, namely deleting the production typedef-name: identifier and making typedef-name a

terminal symbol, this grammar is acceptable to the YACC parser-generator. It has only one

conflict, generated by the if-else ambi-guity.

translation-unit:

external-declaration

translation-unit external-declaration

external-declaration:

function-definition

declaration

function-definition:

declaration-specifiers

,, declarator declaration-list

,, compound-statement

declaration:

declaration-specifiers init -declarator-listopt

declaration-list:

declaration

declaration-list declaration

declaration -specifiers:

storage-class-specifier declaration-specifiersop,

type-specifier declaration-specifiersopt type-

qualifier declaration-specifiers

storage-class-specifier: one of

auto register static extern typedef

type-specifier: one of

void char short int long float double signed unsigned struct-or-union-

specifier · enum-specifier typedef-name

type-qualifier: one of

const volatile

struct -or- union-specifier:

struct-or-union identifier

,, { struct-declaration-list }

struct-or-union identifier

struct-or-union: one of

struct union

struct -declaration -list:

struct -declaration

struct -declaration -list struct -declaration

SECTION Al3 GRAMMAR 235

init -declarator-list:

init -declarator

init-declarator-list , init-declarator

init -declarator:

declarator

declarator = initializer

struct -declaration:

specifier-qualifier-list struct -declarator-list

specifier-qualifier-list:

type-specifier specifier-qualifier-listopt

type-qualifier specifier-qualifier-listopt

struct-declarator-list:

struct-declarator

struct-declarator-list , struct-declarator

struct -declarator:

declarator

declaratoropt : constant-expression

enum -specifier:

enum identifieropt { enumerator-list }

enum identifier

enumerator-list:

enumerator

enumerator-list , enumerator

enumerator:

identifier

identifier = constant-expression

declarator:

pointeropt direct -declarator

direct -declarator:

identifier

( declarator )

direct-declarator [ constant-expressionopt ]

direct-declarator ( parameter-type-list )

direct-declarator ( identifier-listopt )

pointer:

* type-qualifier-listopt

* type-qualifier-listopt pointer

type-qualifier-list:

type-qualifier

type-qualifier-list type-qualifier

parameter-type-list:

parameter-list

parameter-list ,

parameter-list:

parameter-declaration

parameter-list , parameter-declaration

236 REFERENCE MANUAL

parameter-declaration:

declaration-specifiers declarator

declaration -specifiers abstract -declaratoropt

identifier-list:

identifier

identifier-list , identifier

initia/izer:

assignment -expression

{ initializer-list }

{ initializer-list , }

initializer-list:

initializer

initializer-list , initializer

type-name:

specifier-qualifier-list abstract -declaratoropt

abstract -declarator:

pointer .

pointeropt direct-abstract-declarator

direct -abstract -declarator:

( abstract -declarator )

direct -abstract -declaratoropt constant -expressionopt

direct -abstract -declaratoropt parameter-type-listopt

typedef-name:

identifier

statement:

labeled -statement

expression-statement

compound -statement

selection-statement

iteration-statement

jump-statement

labeled -statement:

identifier : statement

case constant-expression statement default

: statement

expression-statement:

expressionop, ;

compound -statement:

{ declaration-listopt statement-listopt }

statement-list:

statement

statement-list statement

selection-statement:

if ( expression ) statement

if ( expression ) statement else statement switch (

expression ) statement

APPENDIX A

SECTION Al3 GRAMMAR 237

iteration-statement:

while ( expression ) statement

do statement while ( expression ) ;

for ( expressionop, ; expressionopt ; expressionopt ) statement

jump -statement:

goto identifier

continue ;

break ;

return expressionopt

expression:

assignment -expression

expression , assignment-expression

assignment -expression:

conditional-expression

unary-expression assignment -operator assignment -expression

assignment-operator: one of

= *= I= %= += -= <<= >>= &= = I=

conditional-expression:

logical-£>~-expression

logical-£>~-expression ? expression : conditional-expression

constant -expression:

conditional-expression

logical·£>~ -expression:

logical-AND-expression

logical-£>~-expression II logical-AND-expression

logical-AND-expression:

inclusive-£>~ -expression

logical-AND-expression && inclusive-£>~-expression

inclusive-()~ -expression:

exclusive-l>~·expression

inclusive-£>~-expression exclusive-£>~-expression

exclusive-£>~ - expression:

AND-expression

exclusive-£>~-expression "AND-expression

AND-expression:

equality-expression

AND-expression & equality-expression

equality-expression:

relational-expression

equality-expression == relational-expression

equality-expression I= relational-expression

relational-expression:

shift -expression

relational-expression < shift -expression

relational-expression > shift-expression

relational-expression < = shift -expression

relational-expression > =shift -expression

238 REFERENCE MANUAL APPENDIX A

shift -expression:

additive-expression

shift-expression << additive-expression

shift-expression >> additive-expression

additive-expression:

multiplicative-expression

additive-expression + multiplicative-expression

additive-expression - multiplicative-expression

multiplicative-expression:

cast -expression

multiplicative-expression * cast-expression

multiplicative-expression I cast-expression

multiplicative-expression %cast-expression

cast-expressiotL·

unary-expression

( type-name ) cast-expression

unary-expression:

postfix-expression

++ unary-expression

-- unary-expression

unary-operator cast -expression

sizeof unary-expression sizeof (

type-name )

unary-operator: one of

&. * +

postfix-expression:

primary-expression

postfix-expression [ expression 1

postfix-expression ( argument-expression-listopt

postfix-expression • identifier postfix-expression ->

identifier postfix-expression ++

postfix-expression --

primary-expressioll·

identifier

constant

string

( expression

argument -expression-list:

assignment -expression

argument-expression-list , assignment-expression

cons tam:

integer-constant

character-constant

floating-constant

enumeration-constant

The following grammar for the preprocessor summarizes the structure of control lines,

but is not suitable for mechanized parsing. It includes the symbol text, which means

ordinary program text, non-conditional preprocessor control lines, or complete preprocessor

conditional constructions.

SECTION Al3 GRAMMAR 239

control-line:

# define identifier token-sequence

# define identifier( identifier , ... , identifier ) token-sequence

# undef identifier

# include <filename>

# include "filename"

# include token-sequence

# line constant "filename"

# 1 ine constant

# error token-sequenceopt

# praqma token-sequenceopt

preprocessor-conditional

preprocessor-conditional:

if-line text elif-parts e/se-partopt # endif if-

line:

# if constant-expression

# ifdef identifier

# ifndef identifier

elif-parts:

elif-line text

e/if-partsop,

e/if-line:

# eli£ constant-expression

else-part:

else-line text

else-line:

# else

APPENDix e: Standard Library

This appendix is a summary of the library defined by the ANSI standard. The

standard library is not part of the C language proper, but an environment that supports

standard C will provide the function declarations and type and macro definitions of this

library. We have omitted a few functions that are of limited utility or easily synthesized

from others; we have omitted multi-byte characters; and we have omitted discussion of

locale issues, that is, properties that depend on local language, nationality, or culture.

The functions, types and macros of the standard library are declared in standard

headers:

<assert.h>

<float.h>

<math.h>

<stdarg.h>

<stdlib.h>

<ctype.h>

<limits.h> <setjmp.h> <stddef.h>

<string.h>

<errno.h>

<locale.h>

<signal.h>

<stdio.h>

<time.h>

A header can be accessed by

#include <header>

Headers may be included in any order and any number of times. A header must be

included outside of any external declaration or definition and before any use of anything

it declares. A header need not be a source file.

External identifiers that begin with an underscore are reserved for use by the library,

as are all other identifiers that begin with an underscore and an upper-case letter or

another underscore.

81. Input and Output: <stdlo.h>

The input and output functions, types, and macros defined in <stdio .·h> represent

nearly one third of the library.

A stream is a source or destination of data that may be associated with a disk or

other peripheral. The library supports text streams and binary streams, although on some

systems, notably UNIX, these are identical. A text stream is a sequence of lines; each line

bas zero or more characters and is terminated by '\n'. An environment may need to

convert a text stream to or from some other representation (such as mapping '\n'to

carriage return and linefeed). A binary stream is a sequence of unprocessed bytes that record

internal data, with the property that if it is written, then read back on the same system, it will

compare equal.

A stream is connected to a file or device by opening it; the connection is broken by

241

242 STANDARD LIBRARY APPENDIX 8

closing the stream. Opening a file returns a pointer to an object of type FILE, which records

whatever information is necessary to control the stream. We will use "file pointer" and

"stream" interchangeably when there is no ambiguity.

When a program begins execution, the three streams stdin, stdout, and stderr are already

open.

B 1. 1 File Operations

The following functions deal with operations on files. The type size t is the unsigned

integral type produced by the sizeof operator.

FILE *fopen(const char *filename, const char *mode)

fopen opens the named file, and returns a stream, or NULL if the attempt fails. Legal

values for mode include

"r 11

11a 11

11r+ 11

W+

11 a+ 11

open text file for reading

create text file for writing; discard previous contents if any

append; open or create text file for writing at end of file

open text file for update (i.e., reading and writing)

create text file for update; discard previous contents if any

append; open or create text file for update, writing at end

Update mode permits reading and writing the same file; fflush or a file-positioning

function must be called between a read and a write or vice versa. If the mode

includes b after the initial letter, as in or "w+b", that indicates a binary file.

Filenames are limited to FILENAME_MAX characters. At most FOPEN_MAX files

may be open at once.

FILE *freopen(const char *filename, const char *mode, FILE

*Stream)

freopen opens the file with the specified mode and associates the stream with it. It

returns stream, or NULL if an error occurs. freopen is normally used to change the files

associated with stdin, stdout, or stderr.

int fflush(FILE *Stream)

On an output stream, fflush causes any buffered but unwritten data to be written; on an

input stream, the effect is undefined. It returns EOF for a wrtte error, and zero otherwise.

fflush(NULL) flushes all output streams.

int fclose(FILE *Stream)

fclose flushes any unwritten data for stream, discards any unread buffered input, frees

any automatically allocated buffer, then closes the stream. It returns EOF if any errors

occurred, and zero otherwise.

int remove(const char *filename)

remove removes the named file, so that a subsequent attempt to open it will fail. It

returns non-zero if the attempt fails.

int rename(const char *Oldname, const char *newname) rename changes the

name of a file; it returns non-zero if the attempt fails.

SECTION Bl INPUT AND OUTPUT: <STDIO.H> 243

FILE *tmpfile(void)

tmpfile creates a

removed when closed

stream, or NULL if it

char *tmpnam(char

temporary file of mode "wb+" that will be automatically or when

the program terminates normally. tmpfile returns a could not

create the file.

s[L_tmpnam])

tmpnam (NULL) creates a string that is not the name of an existing file, and returns a

pointer to an internal static array. tmpnam( s) stores the string in s as well as returning it

as the function value; s must have room for at least L_ tmpnam charac-ters. tmpnam

generates a different name each time it is called; at most TMP _MAX different names are

guaranteed during execution of the program. Note that tmpnam creates a name, not a file.

int setvbuf(FILE *Stream, char *buf, int mode, size_t size)

setvbuf controls buffering for the stream; it must be called before reading, writing, or

any other operation. A mode of _IOFBF causes full buffering, _IOLBF line buffering of

text files, and _ IONBF no buffering. If buf is not NULL, it will be used as the buffer;

otherwise a buffer will be allocated. size determines the buffer size. setvbuf returns non-

zero for any error.

void setbuf(FILE *Stream, char *buf)

If buf is NULL, buffering is turned off for the stream. Otherwise, setbuf is equivalent to

(void) setvbuf ( stream, buf, _ IOFBF, BUFSIZ).

B 1.2 FQrmatted Output

The print£ functions provide formatted output conversion.

int fprintf(FILE *Stream, const char *format, ... )

fprintf converts and writes output to stream under the control of format. The return value

is the number of characters written, or negative if an error occurred.

The format string contains two types of objects: ordinary characters, which are copied to

the output stream, and conversion specifications, each of which causes conver-sion and

printing of the next successive argument to fprintf. Each conversion specifi-cation begins

with the character %and ends with a conversion character. Between the % and the conversion

character there may be, in order:

• Flags (in any order), which modify the specification:

-, which specifies left adjustment of the converted argument in its field.

+, which specifies that the number will always be printed with a sign.

space: if the first character is not a sign, a space will be prefixed.

0: for numeric conversions, specifies padding to the field width with leading zeros.

#, which specifies an alternate output form. For o, the first digit will be zero. For x or x, Ox or OX

will be prefixed to a non-zero result. For e, E, f, g, and G, the output will always have a decimal

point; for g and G, trailing zeros will not be removed.

• A number specifying a minimum field width. The converted argument will be printed in a field at

least this wide, and wider if necessary. If the converted argument has fewer characters than the

field width it will be padded on the left (or right, if left adjustment has been requested) to make

up the field width. The padding character is normally space, but is 0 if the zero padding flag is

present.

244 STANDARD LIBRARY APPENDIX B

• A period, which separates the field width from the precision.

• A number, the precision, that specifies the maximum number of characters to be printed from a

string, or the number of digits to be printed after the decimal point for e, E, or f conversions, or the

number of significant digits for g or G conversion, or the minimum number of digits to be printed

for an integer (leading Os will be added to make up the necessary width).

• A length modifier h, 1 Oetter ell), or L. "h" indicates that the corresponding argument is to be

printed as a short or unsigned short; "1" indicates that the argument is a long or unsigned long; "L"

indicates that the argument is a long double.

Width or precision or both may be specified as *• in which case the value is computed

byconverting the next argument(s), which must be int.

The conversion characters and their meanings are shown in Table 8-1. If the char-acter

after the " is not a conversion character, the behavior is undefined.

TABLE 8-1. PRINTF CONVERSIONS

CHARACTER ARGUMENT TYPE; CONVERTED TO

d, i int; signed decimal notation.

o int; unsigned octal notation (without a leading zero).

x, X int; unsigned hexadecimal notation (without a leading Ox or ox),

using abcdef for Ox or ABCDEF for OX.

u int; unsigned decimal notation.

c int; single character, after conversion to unsigned char.

s char *; characters from the string are printed until a '\0' is reached

or until the number of characters indicated by the pre-

cision have been printed.

f double; decimal notation of the form [- ]mmm.ddd, where the

number of d's is specified by the precision. The default preci-

sion is 6; a precision of 0 suppresses the decimal point.

e, E

double; decimal notation of the form [-]m.dddddde±xx or

[-]m.ddddddE±xx, where the number of d's is specified by the

precision. The default precision is 6; a precision of 0 suppresses

the decimal point.

g, G

double; "e or "E is used if the exponent is less than -4 or

greater than or equal to the precision; otherwise "f is used.

Trailing zeros and a trailing decimal point are not printed.

p void *; print as a pointer (implementation-dependent represen-

tation).

n int *; the number of characters written so far by this call to print£ is

written into the argument. No argument is con-verted.

" no argument is converted; print a "·

int printf(const char *format, ... )

print£ ( ... ) is equivalent to fprintf ( stdout, ... ) .

SECTION 81

INPUT AND OUTPUT: <STDIO.H>

245

int sprintf(char *S, const char *format, ... )

sprint£ is the same as printf except that the output is written into the string s, terminated

with '\0 ' . s must be big enough to hold the result. The return count does not include the

'\0 ' .

vprintf(const char *format, va_list arg)

vfprintf(FILE *Stream, const char *format, va_list arg)

vsprintf(char *S, const char *format, va_list arg)

The functions vprintf, vfprintf, and vsprintf are equivalent to the corresponding printf

functions, except that the variable argument list is replaced by arg, which has been

initialized by the va_start macro and perhaps va_arg calls. See the discussion of

<stdarg. h> in Section B7.

B 1.3 Formatted Input

The scanf functions deal with formatted input conversion.

int fscanf(FILE *Stream, const char *format, •.. )

fscanf reads from stream under control of format, and assigns converted values through

subsequent arguments, each of which must be a pointer. It returns when format is exhausted.

fscanf returns EOF if end of file or an error occurs before any conversion; otherwise it

returns the number of input items converted and assigned.

The format string usually contains conversion specifications, which are used to

direct interpretation of input. The format string may contain:

• Blanks or tabs, which are ignored.

• Ordinary characters (not "), which are expected to match the next non-white space character of the

input stream.

• Conversion specifications, consisting of a X, an optional assignment suppression character *• an

optional number specifying a maximum field width, an optional h, 1, or L indicating the width of

the target, and a conversion character.

A conversion specification determines the conversion of the next input field. Nor-

mally the result is placed in the variable pointed to by the corresponding argument. If

assignment suppression is indicated by *• as in "*S, however, the input field is simply

skipped; no assignment is made. An input field is defined as a string of non-white space

characters; it extends either to the next white space character or until the field width, if

specified, is exhausted. This implies that scanf will read across line boundaries to find its

input, since newlines are white space. (White space characters are blank, tab, new-line,

carriage return, vertical tab, and formfeed.)_

The conversion character indicates the interpretation of the input field. The

corresponding argument must be a pointer. The legal conversion characters are shown in

Table B-2.

The conversion characters d, i, n, o, u, and x may be preceded by h if the argument is

a pointer to short rather than int, or by 1 {letter ell) if the argument is a pointer to long.

The conversion characters e, f, and g may be preceded by 1 if a pointer to double rather

than float is in the argument list, and by L if a pointer to a long double.

246 STANDARD LIBRARY APPENDIX B

TABLE B-2. SCANF CONVERSIONS

CHARACTER

INPUT DATA; ARGUMENT TYPE

d decimal integer; int *·

i integer; int *· The integer may be in octal (leading O) or

hexadecimal (leading Ox or ox).

o octal integer (with or without leading zero); int *·

uunsigned decimal integer; unsigned int *·

xhexadecimal integer (with or without leading Ox or ox); int *·

ccharacters; char *· The next input characters are placed in the indicated

array, up to the number given by the width field; the default is 1.

No '\0' is added. The normal skip over white

space characters is suppressed in this case;

to read the next

non-white space character, use %1 s.

string of non-white space characters (not quoted); char *•

pointing to an array of characters large enough to hold the

string and a terminating '\0' that will be added.

e, f, g

floating-point number; float *· The input format for float's

is an optional sign, a string of numbers possibly containing a

decimal point, and an optional exponent field containing an E or

e followed by a possibly signed integer.

pointer value as printed by printf ( "%p" ); void *·

writes into the argument the number of characters read so far

by this call; int *· No input is read.

The converted item count

is not incremented.

[. .. ]

matches the longest non-empty string of input characters from

the set

between brackets; char *·

A '\0'

is added.

[ ] ... ]

includes

] in the set.

[ " ... ]

matches the longest non-empty string of input characters not

from the set between brackets; char *· A

'\0' is

added.

[ " ] ... ]

includes ] in the set.

% literal %; no assignment is made.

int scanf(const char *format, ... ) scanf (. .. ) is

identical to f scanf ( stdin, ... ) .

int sscanf(char *S, const char *format, .•. )

sscanf ( s, ... ) is equivalent to scanf ( ... ) except that the input characters are taken from

the string s.

B 1.4 Character Input and Output Functions

int fgetc(FILE *Stream)

fgetc returns the next character of stream as an unsigned char (converted to an int), or

EOF if end of file or error occurs.

SECTION 81 INPUT AND OUTPUT: <STDIO.H> 247

char *fgets(char *S, int n, FILE *Stream}

fgets reads at most the next n-1 characters into the array s, stopping if a newline is

encountered; the newline is included in the array, which is terminated by '\0 '. fgets

returns s, or NULL if end of file or error occurs.

int fputc(int c, FILE *Stream}

fputc writes the character c (converted to an unsigned char) on stream. It returns the

character written, or EOF for error.

int fputs(const char *S, FILE *Stream}

fputs writes the string s (which need not contain '\n ') on stream; it returns non-negative,

or EOF for an error.

int getc(FILE *Stream}

getc is equivalent to fgetc except that if it is a macro, it may evaluate stream more than

once.

int getchar(void}

get char is equivalent to getc ( stdin}.

char *gets(char *S}

gets reads the next input line into the array s; it replaces the terminating newline with '\0

'. It returns s, or NULL if end of file or error occurs.

int putc(int c, FILE *Stream}

putc is equivalent to fputc except that if it is a macro, it may evaluate stream more than

once.

int putchar(int c)

putchar ( c} is equivalent to putc ( c, stdout}.

int puts(const char *S}

puts writes the string s and a newline to stdout. It returns EOF if an error occurs, non-

negative otherwise.

int ungetc(int c, FILE *Stream}

ungetc pushes c (converted to an unsigned chad back onto stream, where it will be

returned on the next read. Only one character of pushback per stream is guaranteed.

EOF may not be pushed back. ungetc returns the character pushed back, or EOF for

error.

B 1.5 Direct Input and Output Functions

size_t fread(void *Ptr, size_t size, size_t nobj, FILE *Stream} fread reads from stream int-o

the array ptr at most nobj objects of size size. fread returns the number of objects read;

this may be less than the number requested. feof and ferror must be used to determine

status.

size_t fwrite(const void *Ptr, size_t size, size_t nobj, FILE *Stream)

fwrite writes, from the array ptr, nobj objects of size size on stream. It returns the

number of objects written, which is less than nobj on error.

148 STANDARD LIBRARY APPENDIX 8

B 1.6 File Positioning Functions

int fseek(FILE *Stream, long offset, int origin)

fseek sets the file position for stream; a subsequent read or write will access data

beginning at the new position. For a binary file, the position is set to offset char-acters

from origin, which may be SEEK_SET (beginning), SEEK_CUR (current position), or

SEEK_END (end of file). For a text stream, offset must be zero, or a value returned by

ftell (in which case origin must be SEEK_SET). fseek returns non-zero on error.

long ftell(FILE *Stream)

ftell returns the current file position for stream, or -1L on error.

void rewind(FILE *Stream)

rewind(fp) is equivalent to fseek(fp,OL,SEEK_SET); clearerr(fp).

int fgetpos(FILE *Stream, fpos_t *Ptr)

fgetpos records the current position in stream in *Ptr, for subsequent use by fsetpos. The

type fpos_ t is suitable for recording such values. fgetpos returns non-zero on error.

int fsetpos(FILE *Stream, const fpos_t *Ptr)

fsetpos positions stream at the position recorded by fgetpos in *Ptr.

fsetpos returns non-zero on error.

81.7 Error Functions

Many of the functions in the library set status indicators when error or end of file occur.

These indicators may be set and tested explicitly. In addition, the integer expres-sion errno

(declared in <errno.h>) may contain an error number that gives further information about the

most recent error.

void clearerr(PILE *Stream)

clearerr clears the end of file and error indicators for stream.

int feof(FILE *Stream)

feof returns non-zero if the end of file indicator for stream is set.

int ferror(FILE *Stream)

ferror returns non-zero if the error indicator for stream is set.

void perror(const char *S)

perror ( s) prints s and an implementation-defined error message corresponding to the

integer in errno, as if by

fprintf(stderr, "%s: %s\n", s, "error message")

See strerror in Section B3.

82. Character Class Tests: <ctype.h>

The header <ctype . h> declares functions for testing characters. For each function, the

argument is an int, whose value must be EOF or representable as an unsigned

SECTION 83 STRING FUNCTIONS: <STRING.H> 249

char, and the return value is an int. The functions return non-zero (true) if the argu-ment c

satisfies the condition described, and zero if not.

isalnwn(c)

isalpha (c) or

isdigi t (c) is true

isalpha(c)

isupper (c) or

is lower (c) is true

iscntrl(c)

control character

isdigit(c)

decimal digit

isgraph(c)

printing character except space

islower(c)

lower-case letter

isprint(c)

printing character including space

ispunct(c)

printing character except space or letter or digit

isspace(c)

space, formfeed, newline, carriage return, tab, vertical tab

isupper(c)

upper-case letter

isxdigit(c)

hexadecimal digit

In the seven-bit ASCII character set, the printing characters are Ox20 ('

('-');the control characters are 0 (NUL) to Ox1F (US), and Ox7F (DEL).

In addition, there are two functions that convert the case of letters:

Ox7E

int tolower(int c)

int toupper(int c)

convert c to lower case

convert c to upper case

If c is an upper-case letter, to lower (c) returns the corresponding lower-case letter;

otherwise it returns c. If c is a lower-case letter, toupper (c) returns the correspond-ing

upper-case letter; otherwise it returns c.

83. String Functions: < string.h >

There are two groups of string functions defined in the header <string. h>. The first

have names beginning with str; the second have names beginning with mem. Except for

memmove, the behavior is undefined if copying takes place between overlap-ping objects.

Comparison functions treat arguments as unsigned char arrays.

In the following table, variables s and t are of type char *; cs and ct are of type

const char *; n is of type size_ t; and c is an int converted to char.

char

*Strcpy(s,ct)

copy string ct to string s, including '\0 ';return s.

char

*Strncpy(s,ct,n)

copy at most n characters of string ct to s; return s.

Pad with '\0 's if t has fewer than n characters.

char

*Strcat(s,ct)

concatenate string ct to end of string s; return s.

char

*Strncat(s,ct,n)

concatenate at most n characters of string ct to string

s, terminate s with '\0 ';return s.

int

strcmp(cs,ct)

compare string cs to string ct; return <0 if cs<ct, 0

if cs==ct, or >0 if cs>ct.

int

strncmp(cs,ct,n)

compare at most n characters of string cs to string ct;

return <0 if cs<ct, 0 if cs==ct, or >0 if cs>ct.

char

*Strchr(cs,c)

return pointer to first occurrence of c in cs or NULL if

not present.

char

*Strrchr(cs,c)

return pointer to last occurrence of c _in cs or NULL if

not present.

250

STANDARD LIBRARY

APPENDIX B

size_t

strspn(cs,ct)

return length of prefix of cs consisting of characters in

ct.

size_t

strcspn(cs,ct)

return length of prefix of cs consisting of characters

not in ct.

char

*Strpbrk(cs,ct)

return pointer to first occurrence in string cs of any

character of string ct, or NULL if none are present.

char

*Strstr(cs,ct)

return pointer to first occurrence of string ct in cs, or

NULL if not present.

size_t

strlen(cs)

return length of cs.

char

*Strerror(n)

return pointer to implementation-defined string

corresponding to error n.

char

*Strtok(s,ct)

strtok searches s for tokens delimited by characters

from ct; see below.

sequence

of calls

of strtok ( s, ct) splits s into tokens,

each delimited by a

character from

ct. The

first

call in a sequence has a non-NULL

s. It finds the first

token in s consisting of characters not in ct; it terminates that by overwriting the next character of

s with • \0 • and returns a pointer to the token. Each subsequent call, indi-cated by a NULL value

of s, returns the next such token, searching from just past the end of the previous one. strtok

returns NULL when no further token is found. The string ct may be different on each call.

The mem. .. functions _are meant for manipulating objects as character arrays; the intent is

an interface to efficient routines. In the following table, s and t are of type void *; cs and ct are of

type const void *; n is of type size_ t; and c is an int converted to an unsigned char.

void *memcpy ( s , ct, n) copy n characters from ct to s, and return s.

void *memmove ( s, ct, n) same as memcpy except that it works even if the objects overlap.

int memcmp ( cs, ct, n) compare the first n characters of cs with ct; return as

with strcmp.

void *memchr ( cs, c, n)

void *memset ( s , c , n)

return pointer to first occurrence of character c in cs, or

NULL if not present among the first n characters. place

character c into first n characters of s, return s.

84. Mathematical Functions: <math.h>

The header <math.h> declares mathematical functions and macros.

The macros EDOM and ERANGE (found in <errno.h>) are non-zero integral con-stants that

are used to signal domain and range errors for the functions; HUGE_ VAL is a positive double

value. A domain error occurs if an argument is outside the domain over which the function is

defined. On a domain error, errno is set to EDOM; the return value is implementation-dependent.

A range error occurs if the result of the function cannot be represented as a double. If the result

overflows, the function returns HUGE_ VAL with the right sign, and errno is set to ERANGE. If

the result underflows, the function returns zero; whether errno is set to ERANGE is

implementation-defined.

In the following table, x and y are of type double, n is an int, and all functions return double.

Angles for trigonometric functions are expressed in radians.

SECTION 85

UTILITY FUNCTIONS: <STDLIB.H>

251

sin(x)

sine of x

cos(x)

cosine of x

tan(x)

tangent of x

asin(x)

sin-

(x) in range [-1r/2, 11"/2], x

E [-1, 11.

acos(x)

cos-1 (x) in range [0, 1r], x E [-1, 1].

atan(x)

tan- 1 (x) in range [-11"/2, 1r/2l

atan2 ( y, x)

tan- 1 (y!x) in range [ -1r, 11"].

sinh ( x)

hyperbolic sine of x

cosh(x)

hyperbolic cosine of x

tanh(x)

hyperbolic tangent of x

exp(x)

exponential function ex

log(x)

natural logarithm ln(x), x>O.

log10(x)

base 10 logarithm log

(x), x>O.

pow(x,y)

xY. A domain error occurs if

x-0 and y~O. or if

x < 0 and y is not an integer.

sqrt(x)

£, x~O.

ceil (x)

smallest integer not less than x, as a double.

floor(x)

largest integer not greater than x, as a double.

fabs(x)

absolutevalue lxl

ldexp(x,n) x·2"

frexp(x, int *exp)

splits x into a normalized fraction in the interval [112, 1),

which is returned, and a power of 2, which is stored in

*exp. If x is zero, both parts of the result are zero.

modf(x, double *iP)

splits x into integral and fractional parts, each with the

same sign as x. It stores the integral part in *ip, and

returns the fractional part.

fmod ( x, y) floating-point remainder of x/y, with the same sign as

x. If y is zero, the result is implementation-defined.

85. Utility Functions: < stdlib.h >

The header <stdlib. h> declares functions for number conversion, storage alloca-tion, and

similar tasks.

double atof(con8t char *8)

atof converts 8 to double; it is equivalent to strtod(s, (char**)NULL).

int atoi(con8t char *S)

converts s to int; it is equivalent to ( int) strtol ( s, (char**) NULL, 1 0 ) .

long atol(con8t char *8)

converts 8 to long; it is equivalent to strtol ( s, (char** )NULL, 10 ).

double 8trtod(const char *S, char **endp)

strtod converts the prefix of s to double, ignoring leading white space; it stores a point~r to

any unconverted suffix in *endp unless endp is NULL. If the answer

151 STANDARD LIBRARY APPENDlX B

would overflow, HUGE_ VAL is returned with the proper sign; if the answer would

underflow, zero is returned. In either case errno is set to ERANGE.

long strtol(const char *S, char **endp, int base)

strtol converts the prefix of s to long, ignoring leading white space; it stores a pointer to

any unconverted suffix in *endp unless endp is NULL. If base is between 2 and 36,

conversion is done assuming that the input is written in that base. If base is zero, the base

is 8, 10, or 16; leading 0 implies octal and leading Ox or OX hexadecimal. Letters in

either case represent digits from 10 to base-1; a leading Ox or OX is permitted in base 16.

If the answer would overflow, LONG _MAX or LONG_MIN is returned, depending on

the sign of the result, and errno is set to

ERANGE.

unsigned long strtoul(const char *S, char **endp, int base) strtoul is the same as strtol except

that the result is unsigned long and the

error value is ULONG _MAX.

int rand(void)

rand returns a pseudo-random integer in the range 0 to RAND _MAX, which is at least

32767.

void srand(unsigned int seed)

srand uses seed as the seed for a new sequence of pseudo-random numbers. The initial

seed is 1.

void *Calloc(size_t nobj, size_t size)

calloc returns a pointer to space for an array of nobj objects, each of size size, or NULL if

the request cannot be satisfied. The space is initialized to zero bytes.

void *malloc(size_t size)

malloc returns a pointer to space for an object of size size, or NULL if the request cannot

be satisfied. The space is uninitialized.

void *realloc(void *P, size_t size)

realloc changes the size of the object pointed to by p to size. The contents will be

unchanged up to the minimum of the old and new sizes. If the new size is larger, the new

space is uninitialized. realloc returns a pointer to the new space, or NULL if the request

cannot be satisfied, in which case *P is unchanged.

void free(void *P)

free deallocates the space pointed to by p; it does nothing if p is NULL. p must be a

pointer to space previously allocated by calloc, malloc, or realloc.

void abort(void)

abort causes the program to terminate abnormally, as if by raise ( SIGABRT).

void exit(int status)

exit causes normal program termination. atexi t functions are called in reverse order of

registration, open files are flushed, open streams are closed, and control is returned to the

environment. How status is returned to the environment is implementation-dependent,

but zero is taken as successful termination. The values

EXIT_SUCCESS and EXIT_FAILURE may also be used.

SECTION 86 DIAGNOSTICS: <ASSERT.H>

153

int atexit(void (*fcn)(void))

atexit registers the function fen to be called when the

mally; it returns non-zero if the registration cannot be made.

program terminates nor-

int system(const char *S)

system passes the string s to the environment for execution. If s is NULL, system

returns non-zero if there is a command processor. If s is not NULL, the return value is

implementation-dependent.

char *getenv(const char *name)

getenv returns the environment string associated with name, or NULL if no string

exists. Details are implementation-dependent.

void *bsearch(const void *key, const void *base, size_t n,

size_t size,

int (*cmp)(const void *keyval, const void *datum))

bsearch searches base [ 0] ... base [ n-1 ] for an item that matches *key. The

function cmp must return negative if its first argument (the search key) is less than

its second (a table entry), zero if equal, and positive if greater. Items in the array

base must be in ascending order. bsearch returns a pointer to a matching item,

or NULL if none exists.

void qsort(void *base, size_t n, size_t size,

int (*cmp)(const void*' const void*))

qsort sorts into ascending order an array base[ 0 ] ... base[n-1] of objects of size size.

The comparison function cmp is as in bsearch.

int abs(int n)

abs returns the absolute value of its int argument.

long labs(long n)

labs returns the absolute value of its long argument.

div_t div(int num, int denom)

div computes the quotient and remainder of num/denom. The results are stored in the

int members quot and rem of a structure of type div _t.

ldiv_t ldiv(long num, long denom)

div computes the quotient and remainder of num/denom. The results are stored in the

long members quot and rem of a structure of type ldiv _ t.

86. Diagnostics: <assert.h>

The assert macro is used to add diagnostics to programs:

void assert ( int expression)

If expression is zero when

assert (expression )

is executed, the assert macro will print on stderr a message, such as

Assertion failed: expression, file filename, line nnn

It then calls abort to terminate execution. The source filename and line number come

254 STANDARD LIBRARY APPENDIX B

from the preprocessor macros __ FILE __ and __ LINE __ .

If NDEBUG is defined at the time <assert. h> is included, the assert macro is ignored.

87. Variable Argument Usts: < stdarg.h>

The header <stdarg. h> provides facilities for stepping through a list of function

arguments of unknown number and type.

Suppose lastarg is the last named parameter of a function f with a variable number of

arguments. Then declare within f a variable ap of type va_list that will point to each

argument in turn:

va_list ap;

ap must be initialized once with the macro va_start before any unnamed argument is

accessed:

va_start(va_list ap, lastarg);

Thereafter, each execution of the macro va_arg will produce a value that has the type and

value of the next unnamed argument, and will also modify ap so the next use of va_arg

returns the next argument:

type va_arg(va_list ap, type); The macro

void va_end(va_list ap);

must be called once after the arguments have been processed but before f is exited.

88. Non-local Jumps: < setjmp.h>

The declarations in <set jmp. h> provide a way to avoid the normal function call and

return sequence, typically to permit an immediate return from a deeply nested func- · tion call.

int setjmp(jmp_buf env)

The macro setjmp saves state information in env for use by longjmp. The return is zero

from a direct call of setjmp, and non-zero from a subsequent call of longjmp. A call to

setjmp can only occur in certain contexts, basically the test of if, switch, and loops, and

only in simple relational expressions.

if (setjmp(env) == 0)

/* get here on direct call */

else

/* get here by calling longjmp */

void longjmp(jmp_buf env, int val)

longjmp restores the state saved by the most recent call to setjmp, using informa-tion

saved in env, and execution resumes as if the setjmp function had just exe-cuted and

returned the non-zero value val. The function containing the set jmp must not have

terminated. Accessible objects have the values they had when longjmp was called,

except that non-volatile automatic variables in the function calling setjmp become

undefined if they were changed after the setjmp call.

SECTION 810 DATE AND TIME FUNCTIONS: <TIME.H> 255

89. Signals: < signal.h>

The header <signal. h> provides facilities for handling exceptional conditions that arise

during execution, such as an interrupt signal from an external source or an error in

execution.

void (*signal(int sig, void (*handler)(int)))(int)

signal determines how subsequent signals will be handled. If handler is SIG_DFL, the

implementation-defined default behavior is used; if it is SIG_IGN, the signal is ignored;

otherwise, the function pointed to by handler will be called, with the argument of the type of

signal. Valid signals include

SIGABRT abnormal termination, e.g., from abort

SIGFPE arithmetic error, e.g., zero divide or overflow

SIGILL illegal function image, e.g., illegal instruction

SIGINT interactive attention, e.g., interrupt

SIGSEGV illegal storage access, e.g., access outside memory limits

SIGTERM termination request sent to this program

signal returns the previous value of handler for the specific signal, or SIG_ERR if an error occurs.

When a signal sig subsequently occurs, the signal is restored to its default behavior; then

the signal-handler function is called, as if by (*handler) ( sig). If the handler returns, execution

will resume where it was when the signal occurred.

The initial state of signals is implementation-defined.

int raise(int sig)

raise sends the signal sig to the program; it returns non-zero if unsuccessful.

81 0. Date and Time Functions: < time.h>

The header <time. h> declares types and functions for manipulating date and time. Some

functions process local time, which may differ from calendar time, for example

because of time zone. clock_ t

and time_ t are arithmetic types representing times,

and struct tm holds the components of a calendar time:

int

tm_sec;

seconds after the minute (0, 61)

int

tm_min;

minutes after the hour (0, 59)

int

tm_hour;

hours since midnight (0, 23)

int

tm_mday;

day of the month (I, 31)

int

tm_mon;

months since January (0, II)

int

tm_year;

years since 1900

int

tm_wday;

days since Sunday (0, 6)

int

tm_yday;

days since January I (0, 365)

int

tm_isdst;

Daylight Saving Time flag

tm_isdst is positive if Daylight Saving Time is in effect, zero if not, and negative if the

information is not available.

clock_t clock(void)

clock returns the processor time used by the program since the beginning of execu-tion, or -1

if unavailable. clock ( ) /CLOCKS_PER_SEC is a time in seconds.

256 STANDARD LIBRARY APPENDIX B

time_t time(time_t *tp)

time returns the current calendar time or -1 if the time is not available. If tp is not NULL,

the return value is also assigned to *tp.

double difftime(time_t time2, time_t time1) difftime returns

time2-time 1 expressed in seconds.

time_t mktime(struct tm *tp)

mktime converts the local time in the structure *tP into calendar time in the same

representation used by time. The components will have values in the ranges shown.

mktime returns the calendar time or -1 if it cannot be represented.

The next four functions return pointers to static objects that may be overwritten by

other calls.

char *asctime(const struct tm *tP)

asctime converts the time in the structure *tp into a string of the form Sun

Jan 3 15:14:13 1988'n'O

char *Ctime(const time_t *tp)

ctime converts the calendar time *tP to local time; it is equivalent to

asctime(localtime(tp))

struct tm *gmtime(const time_t *tp)

gmtime converts the calendar time *tP into Coordinated Universal Time (UTC). It

returns NULL if UTC is not available. The name gmtime has historical significance.

struct tm *localtime(const time_t *tp) localtime converts the

calendar time *tP into local time.

size_t strftime(char *S, size_t smax, const char *fmt, const struct tm

*tp)

strftime formats date and time information from *tP into s according to fmt, which is

analogous to a print£ format. Ordinary characters (including the ter-minating ''0 ') are

copied into s. Each %c is replaced as described below, using values appropriate for the

local environment. No more than smax characters are placed into s. strftime returns the

number of characters, excluding the ''0 ', or zero if more than smax characters were

produced.

abbreviated weekday name.

full weekday name.

abbreviated month name.

full month name.

local date and time representation.

day of the month (01-31).

hour (24-hour clock)

(00-23).

hour (12-hour clock)

(01-12).

day of the year (001-366).

SECTION 811

%M.

»l

IMPLEMENTATION-DEFINED LIMITS: <LIMITS.H> AND <FLOAT.H> 257

month (01-12).

minute (00-59).

local equivalent of AM or PM.

second (00-61).

week number of the year (Sunday as 1st day of week) (00-53).

weekday (0-6, Sunday is 0).

week number of the year (Monday as 1st day of week) (00-53).

local date representation.

local time representation.

year without century (00-99).

year with century.

time zone name, if any.

B 11. Implementation-defined Limits: < limits.h > and < float.h >

The header <1 imi ts . h> defines constants for the sizes of integral types. The values below

are acceptable minimum magnitudes; larger values may be used.

CHAR_BIT

bits in a char

CHAR_MAX

UCHAR_MAX or

SCHAR_MAX

maximum value of char

CHAR_MIN

0 or SCHAR_MIN

minimum value of char

INT_MAX

+32767

maximum value of int

INT_MIN

-32767

minimum value of int

LONG_MAX

+2147483647

maximum value of long

LONG_MIN

-2147483647

minimum vl'tlue of long

SCHAR_MAX

+127

maximum value of signed char

SCHAR_MIN

-127

minimum value of signed char

SHRT_MAX

+32767

maximum value of short

SHRT_MIN

-32767

minimum value of short

UCHAR_MAX

255

maximum value of unsigned char

UINT_MAX

65535

maximum value of unsigned int

ULONG_MAX

4294967295

maximum value of unsigned long

USHRT_MAX

65535

maximum value of unsigned short

The names in the table below, a subset of <float. h>, are constants related to floating-point

arithmetic. When a value is given, it represents the minimum magnitude for the corresponding

quantity. Each implementation defines appropriate values.

FLT_RADIX

radix of exponent representation, e.g., 2, 16

FLT_ROUNDS

floating-point rounding mode for addition

FLT_DIG

decimal digits of precision

FLT_EPSILON

1E-5

smallest number x such that 1.0 + x ¢ 1.0

FLT_MANT_DIG

number of base FLT _RADIX digits in mantissa

FLT_MAX

1E+37

maximum floating-point number

FLT_MAX_EXP

maximum n such that FLT _RADIXn-1 is representable

FLT_MIN

1E-37

minimum normalized floating-point number

FLT_MIN_EXP

minimum n such that 10n is a normalized number

258 STANDARD LIBRARY APPENDIX B

DBL_DIG

1 0

decimal digits of precision

DBL_EPSILON

1E-9

smallest number x such that 1.0 + x ~ 1.0

DBL_MANT_DIG

number of base FLT_RADIX digits in mantissa

DBL_MAX

1E+37

maximum double floating-point number

DBL_MAX_EXP

maximum n such that FLT _RADI:x"-1 is representable

DBL_MIN

1E-37

minimum normalized double floating-point number

DBL_MIN_EXP

minimum n such that 10" is a normalized number

APPENDIX c: Summary of Changes

Since the publication of the first edition of this book, the definition of the C language

has undergone changes. Almost all were extensions of the original language, and were

carefully designed to remain compatible with existing practice; some repaired ambiguities in

the original description; and some represent modifications that change existing practice.

Many of the new facilities were announced in the documents accom-panying compilers

available from AT & T, and have subsequently been adopted by other suppliers of C

compilers. More recently, the ANSI committee standardizing the language incorporated most

of these changes, and also introduced other significant modifications. Their report was in

part anticipated by some commercial compilers even before issuance of the formal C

standard.

This Appendix summarizes the differences between the language defined by the first

edition of this book, and that expected to be defined by the final Standard. It treats only the

language itself, not its environment and library; although these are an important part of the

Standard, there is little to compare with, because the first edition did not attempt to prescribe

an environment or library.

• Preprocessing is more carefully defined in the Standard than in the first edition, and is

extended: it is explicitly token based; there are new operators for catenation of tokens

(##), and creation of strings (#); there are new control lines like #elif and #pragma;

redeclaration of macros by the same token sequence is explicitly permit-ted; parameters

inside strings are no longer replaced. Splicing of lines by \ is per-mitted everywhere, not

just in strings and macro definitions. See §A12.

• The minimum significance of all internal identifiers is increased to 31 characters; the

smallest mandated significance of identifiers with external linkage remains 6 mono-case

letters. (Many implementations provide more.)

• Trigraph sequences introduced by ? ? allow representation of characters lacking in some

character sets. Escapes for #\" [ ] { } : - are defined; see §A 12.1. Observe that the

introduction of trigraphs may change the meaning of strings containing the sequence ? ?

• New keywords (void, const, volatile, signed, enum> are introduced. The stillborn entry

keyword is withdrawn.

• New escape sequences, for use within character constants and string literals, are defined.

The effect of following \ by a character not part of an approved escape sequence is

undefined. See §A2.5.2.

259

l60 SUMMARY OF CHANGES APPENDIX C

• Everyone's favorite trivial change: 8 and 9 are not octal digits.

• The Standard introduces a larger set of suffixes to make the type of constants expli-cit: u

or L for integers, F or L for floating. It also refines the rules for the type of unsuffixed

constants (§A2.5).

• Adjacent string literals are concatenated.

• There is a notation for wide-character string literals and character constants; see §A2.6.

• Characters; as well as other types, may be explicitly declared to carry, or not to carry, a

sign by using the keywords signed or unsigned. The locution long float as a synonym for

double is withdrawn, but long double may be used to declare an extra-precision floating

quantity.

• For some time, type unsigned char has been available. The standard introduces the signed

keyword to make signedness explicit for char and other integral objects.

• The void type has been available in most implementations for some years. The Standard

introduces the use of the void * type as a generic pointer type; previously char * played

this role. At the same time, explicit rules are enacted against mixing pointers and

integers, and pointers of different type, without the use of casts.

• The Standard places explicit minima on the ranges of the arithmetic types, and man-

dates headers (<limits.h> and <float.h>) giving the characteristics of each particular

implementation.

• Enumerations are new since the first edition of this book.

• The Standard adopts from C++ the notion of type qualifier, for example const (§A8.2).

• Strings are no longer modifiable, and so may be placed in read-only memory.

• The "usual arithmetic conversions" are changed, essentially from "for integers, unsigned

always wins; for floating point, always use double" to "promote to the smallest

capacious-enough type." See §A6.5.

• The old assignment operators like = + are truly gone. Also, assignment operators are now

single tokens; in the first edition, they were pairs, and could be separated by white space.

• A compiler's license to treat mathematically associative operators as computationally

associative is revoked.

• A unary +operator is introduced for symmetry with unary -.

• A pointer to a function may be used as a function designator without an explicit *

operator. See §A7.3.2.

• Structures may be assigned, passed to functions, and returned by functions.

• Applying the address-of operator to arrays is permitted, and the result is a pointer to the

array.

• The sizeof operator, in the first edition, yielded type int; subsequently, many

implementations made it unsigned. The Standard makes its type explicitly

implementation-dependent, but requires the type, size_ t, to be defined in a

SUMMARY OF CHANGES APPENDIX C 261

standard header (<stddef .h>). A similar change occurs in the type (ptrdiff_t) of the

difference between pointers. See §A7.4.8 and §A7.7.

• The address-of operator &. may not be applied to an object declared register, even if

the implementation chooses not to keep the object in a register.

• The type of a shift expression is that of the left operand; the right operand can't pro-

mote the result. See §A 7 .8.

• The Standard legalizes the creation of a pointer just beyond the end of an array, and

allows arithmetic and relations on it; see §A 7. 7.

• The Standard introduces (borrowing from C++) the notion of a function prototype

declaration that incorporates the types of the parameters, and includes an explicit

recognition of variadic functions together with an approved way of dealing with

them. See §§A7.3.2, A8.6.3, B7. The older style is still accepted, with restrictions.

• Empty declarations, which have no declarators and don't declare at least a structure,

union, or enumeration, are forbidden by the Standard. On the other hand, a declara-

tion with just a structure or union tag redeclares that tag even if it was declared in an

outer scope.

• External data declarations without any specifiers or qualifiers (just a naked declara-

tor) are forbidden.

• Some implementations, when presented with an extern declaration in an inner block,

would export the declaration to the rest of the file. The Standard makes it clear that

the scope of such a declaration is just the block.

• The scope of parameters is injected into a function's compound statement, so that

variable declarations at the top level of the function cannot hide the parameters.

• The name spaces of identifiers are somewhat different. The Standard puts all tags in

a single name space, and also introduces a separate name space for labels; see §All.l.

Also, member names are associated with the structure or union of which they are a

part. (This has been common practice from some time.)

• Unions may be initialized; the initializer refers to the first member.

• Automatic structures, unions, and arrays may be initialized, albeit in a restricted

way.

• Character arrays with an explicit size may be initialized by a string literal with

exactly that many characters (the \0 is quietly squeezed out).

• The controlling expression, and the case labels, of a switch may have any integral

type.

•

0 ... octal constant 37, 193

Ox ... hexadecimal constant 37, 193

+ addition operator 41, 205

& address operator 93, 203

= assignment operator 17, 42, 208

+= assignment operator 50

\\ backslash character 8, 38

& bitwise AND operator 48, 207

A bitwise exclusive OR operator 48, 207

: bitwise inclusive OR operator 48, 207

, comma operator 62, 209

? : conditional expression 51, 208

• .. declaration 155, 202

--decrement operator 18, 46, 106, 203

I division operator 10, 41, 205

==equality operator 19, 41, 207

>=greater or equal operator 41, 206 >

greater than operator 41, 206

++ increment operator 18, 46, 106, 203

* indirection operator 94, 203

I= inequality operator 16, 41, 207

« left shift operator 49, 206

<=less or equal operator 41, 206

<less than operator 41, 206

&& logical AND operator 21, 41, 49, 207 I

logical negation operator 42, 203-204

:: logical OR operator 21, 41, 49, 208

% modulus operator 41, 205

*multiplication operator 41, 205

- one's complement operator 49, 203-204

# preprocessor operator 90, 230

## preprocessor operator 90, 230

' quote character 19, 37-38, 193 n

quote character 8, 20, 38, 194 »

right shift operator 49, 206

• structure member operator 128, 201 -

>structure pointer operator 131, 201 -

subtraction operator 41, 205

-unary minus operator 203-204 +

unary plus operator 203-204

_ underscore character 35, 192, 241

\0 null character 30, 38, 193

\a alert character 38, 193

abort library function 252

Index

abs library function 253

abstract declarator 220

access mode, file 160, 178, 242

acos library function 251

actual argument see argument

addition operator,+ 41, 205

additive operators 205

addpoint function 130

address arithmetic see pointer arithmetic

address of register 21 0

address of variable 28, 94, 203

address operator, & 93, 203

addtree function I 41 afree

function 102

alert character, \a 38, 193

alignment, bit-field 150, 213

alignment by union 186

alignment restriction 138, 142, 148, 167, 185,

199

alloc function 101

allocator, storage 142, 185-189

ambiguity, if-else 56, 223, 234

American National Standards Institute (ANSI)

ix, 2, 191

a.out 6, 70

argc argument count 114

argument, definition of 25, 201

argument, function 25, 202

argument list, variable length 155, 174, 202,

218, 225, 254

argument list, void 33, 73, 218, 225

argument, pointer I 00

argument promotion 45, 202

argument, subarray 100 arguments,

command-line 114-118 argv argument

vector 114, 163 arithmetic conversions,

usual 42, 198 arithmetic operators 41

arithmetic, pointer 94, 98, 100-103, 117, 138,

205

arithmetic types 196

array, character 20, 28, 104

array declaration 22, Ill, 216

array declarator 216

array initialization 86, 113, 219

array, initialization of two-dimensional 112,

220

263

264 THE C PROGRAMMING LANGUAGE

array, multi-dimensional

I 10, 217

array name argument

28, I 00, 112

array name, conversion of

99, 200

array of pointers

107

array reference 201

array size, default

86,

113, 133

array, storage order of

112, 217

array subscripting

22, 97, 201, 217

array, two-dimensional

II 0, 112, 220

array vs. pointer

97, 99-100,

I 04, 113

arrays of structures

132

ASCII character set

19, 37, 43, 229, 249

asctime library function

256

asin library function

251

asm keyword

192

<assert. h> header

253

assignment, conversion by

44, 208

assignment expression

17, 21, 51, 208

assignment, multiple 21

assignment operator, = 17, 42, 208

assignment operator, +=

assignment operators 42, 50, 208

assignment statement, nested

I 7, 21, 51

assignment suppression, scanf

157, 245

associativity of operators

52, 200

atan, atan2 library functions

251

atexi t library function

253

atof function

atof library function

251

atoi function

43, 61, 73

a toi library function

251

atol library function

251

auto storage class specifier 210

automatic storage class

31, 195

automatic variable

31, 74, 195

automatics, initialization of

31, 40, 85, 219

automatics, scope of 80, 228

avoiding goto 66

\b backspace character

8, 38, 193

backslash character, \\

8, 38

bell character

see alert character

binary stream

160, 241-242

binary tree 139

binsearch function

58, 134, 137

bit manipulation idioms

49, 149

bi tcount function 50

bit-field alignment

150, 213

bit-field declaration

150, 212

bitwise AND operator,&. 48, 207

bitwise exclusive OR operator, "

48, 207

bitwise inclusive OR operator,

48, 207

bitwise operators

48, 207

block see compound statement

block, initialization in

84, 223

block structure

55, 84, 223

boundary condition

19, 65

braces 7, 10, 55, 84

braces, position of

break statement

59, 64, 224

bsearch library function

253

buffered getchar

172

buffered input

170

INDEX

buffering see setbuf, setvbuf

BUFSIZ 243

calculator program 72, 74, 76, 158

call by reference 27

call by value 27, 95, 202

calloc library function 167, 252

canonrect function 131

carriage return character, \r 38, 193 case

label 58, 222

cast, conversion by 45, 198-199, 205 cast

operator 45, 142, 167, 198, 205, 220 cat

program 160, 162-163

cc command 6, 70

ceil library function 251 char

type I 0, 36, 195, 211 character

array 20, 28, 104 character

constant 19, 37, 193 character

constant, octal 37 character

constant, wide 193 character

count program 18 character

input/output 15, 151 character set

229

character set, ASCII 19, 37, 43, 229, 249

character set, EBCDIC 43

character set, ISO 229 character, signed 44,

195 character string see string constant

character testing functions 166, 248 character,

unsigned 44, 195 character-integer conversion

23, 42, 197 characters, white space 157, 166,

245, 249 clearerr library function 248

CLOCKS PER SEC 255

clock library-function 255

clock_ t type name 255 close

system call 174 closedir

function 184 coercion see

cast

comma operator, , 62, 209

command-line arguments 114-118

comment 9, 191-192, 229

comparison, pointer I 02, 138, 187, 207

compilation, separate 67, 80, 227

compiling a C program 6, 25

compiling multiple files 70

compound statement 55, 84, 222, 225-226

concatenation, string 38, 90, 194

concatenation, token 90, 230 conditional

cvmpilation 91, 231 conditional expression, ?

: 51, 208 const qualifier 40, 196,l2JI

constant expression 38, 58, 91, 209

constant, manifest 230 constant suffix

37, 193

constant, type of 37, 193

constants 37, 192

continue statement 65, 224

control character 249

control line 88, 229-233

conversion 197-199

conversion by assignment 44, 208

conversion by cast 45, 198-199, 205

THE C PROGRAMMING LANGUAGE

conversion by return

73, 225

conversion, character-integer

23, 42, 197

conversion, double-float

45, 198

conversion, float-double

44, 198

conversion, floating-integer

45,

197

conversion, integer-character

conversion, integer-floating

12, 197

conversion, integer-pointer

199, 205

conversion of array name 99, 200

conversion of function

200

see

cast

conversion operator, explicit

conversion, pointer 142, 198, 205

conversion, pointer-integer

198-199, 205

conversions, usual arithmetic

42, 198

copy function

29, 33

cos library function

251

cosh library function

251

creat system call 172

CRLF 151, 241

ctime library function

256

<ctype. h> header

43, 248

date conversion Ill

day_ of _year function

Ill

del function

123

del program

125

declaration 9, 40, 210--218

declaration, array 22, 111, 216

declaration, bit-field

150, 212

declaration, external

225-226

declaration of external variable

31, 225

declaration of function

217-218

declaration of function, implicit

27, 72, 201

declaration of pointer

94, 100, 216

declaration, storage class 210

declaration, structure

128, 212

declaration, type 216

declaration, typedef

146, 210, 221

declaration, union

147, 212

declaration vs. definition

33, 80, 210

declarator 215-218

declarator, abstract

220

declarator, array 216

declarator, function

217

decrement operator,--

18, 46, 106, 203

default array size 86, 113, 133

default function type

30, 201

default initialization

86, 219

default label 58, 222

57, 59

defensive programming

#define 14,89, 229

#define, multi-line

#define vs. enum 39, 149

#define with arguments

defined preprocessor operator

91, 232

definition, function 25, 69, 225

definition, macro 229

definition of argument

25, 201

definition of external variable

33, 227

definition of parameter

25, 201

definition of storage

21 0

definition, removal of see #undef

definition, tentative

227

INDEX

265

dereference see indirection

derived types

l 0, 196

descriptor, file

170

designator, function

201

difftime library function 256

DIR structure

180

dirdcl function 124

directory list program

179

Dirent structure 180

dir. h include file

183

dirwalk function

182

div library function

253

division, integer

l 0,

division operator, I

10, 41, 205

div_t, ldiv_t type names 253

do statement

63, 224

do-nothing function

double constant 37, 194

double type

10, 18, 36, 196, 211

double-float conversion 45, 198

E notation 37,

194

EBCDIC character set

echo program

115-116

EDOM 250

efficiency 51, 83, 88, 142, 187

else see if-else statement

#else, #elif 91,232

else-if 23, 57

empty function

empty statement

see null statement

empty string 38

end of file see EOF

#endif

enum specifier

39, 215

enum vs. #define

39, 149

enumeration constant

39, 91, 193-194, 215

enumeration tag

215

enumeration type

196

enumerator 194, 215

EOF 16, 151, 242

equality operator,==

19, 41, 207

equality operators . 41, 207

equivalence, type

221

ERANGE

250

errno

248, 250

<errno. h> header

248

#error

233

error function

174

errors, input/output

164, 248

escape sequence

8, 19, 37-38, 193, 229

escape sequence, \x hexadecimal 37, 193

escape sequences, table of 38, 193

evaluation, order of

21, 49, 53, 63, 77, 90, 95,

200

exceptions 200, 255

exit library function 163, 252

EXIT FAILURE, EXIT SUCCESS 252

exp library function 251

expansion, macro 230

explicit conversion operator see cast

exponentiation 24, 251 expression

200--209

266 THE C PROGRAMMING LANGUAGE

expression, assignment 17, 21, 51, 208

expression, constant 38, 58, 91, 209

expression order of evaluation 52, 200

expression, parenthesized 201

expression, primary 200

expression statement 55, 57, 222

extern storage class specifier 31, 33, 80, 210

external declaration 225-226

external linkage 73, 192, 195, 211, 228

external names, length of 35, 192 external

static variables 83 external variable 31,

73, 195

external variable, declaration of 31, 225

external variable, definition of 33, 227

externals, initialization of 40, 81, 85, 219

externals, scope of 80, 228

\f formfeed character 38, 193 fabs

library function 251 fclose library

function 162, 242 f cntl • h include

file 172

feof library function 164, 248

feof macro 176

ferror library function 164, 248

ferror macro 176

fflush library function 242 fgetc

library function 246 fgetpos

library function 248 fgets

function 165

fgets library function 164, 247

field see bit-field

file access 160, 169, 178, 242 file

access mode 160, 178, 242 file

appending 160, 175, 242 file

concatenation program 160 file copy

program 16-17, 171, 173

file creation

161, 169

file descriptor

170

file inclusion

88, 231

file opening

160, 169, I 72

file permissions

17 3

file pointer 160, 175, 242

__ FILE __ preprocessor name 254

FILE type name 160 filecopy

function 162 filename suffix, . h 33

FILENAME MAX 242

f illbuf -function 178 float constant

37, 194 float type 9, 36, 196, 211

float-double conversion 44, 198

<float.h> header 36, 257 floating

constant 12, 37, 194 floating point,

truncation of 45, 197 floating types

196 floating-integer conversion 45,

197 floor library function 251

fmod library function 251

fopen function 177

fopen library function 160, 242

FOPEN MAX 242

for ( ; ; ) infinite loop 60, 89 for

statement 13, 18, 60, 224

INDEX

for vs. while 14, 60

formal parameter see parameter

formatted input see scanf

formatted output see printf

formfeed character, \f 38, 193

fortran keyword l92

fpos _ t type name 248

fprintf library function 161, 243 fputc

library function 247 fputs function

165

fputs library function 164, 247

fread library function 247 free

function 188

free library function 167, 252 freopen

library function 162, 242 frexp library

function 251 fscanf library function

161, 245 fseek library function 248

fsetpos library function 248 fsize

function 182

fsize program 181 fstat

system call I 83 ftelllibrary

function 248 function

argument 25, 202

function argument conversion see argument

promotion

function call semantics 201

function call syntax 20 I function,

conversion of 200 function,

declaration of 217-218 function

declaration, static 83 function

declarator 21 7 function definition

25, 69, 225 function designator 201

function, implicit declaration of 27, 72, 201

function names, length of 35, 192

function, new-style 202

function, old-style 26, 33, 72, 202

function, pointer to 118, 147, 201

function prototype 26, 30, 45, 72, 120, 202

function type, default 30, 201

functions, character testing 166, 248

fundamental types 9, 36, 195

fwri te library function 247

generic pointer see void * pointer

getbi ts function 49

getc library function 161, 247

getc macro 176

getch function 79

getchar, buffered 172

getchar library function 15, 151, 161, 247

getchar, unbuffered 171

getenv library function 253

getint function 97

getline function 29, 32, 69, 165

getop function 78

gets library function 164, 247

gettoken function 125 getword

function I 36 gmtime library

function 256 goto statement 65,

224

greater or equal operator, >= 41, 206

THE C PROGRAMMING LANGUAGE

greater than operator, > 41, 206

• h filename suffix 33

hash function 144 hash

table 144 header file

33, 82

headers, table of standard 241 hexadecimal

constant, Ox... 37, 193 hexadecimal escape

sequence, \.x 37, 193 Hoare, C. A. R. 87

HUGE_ VAL 250

identifier 192

#if 91, 135, 231

#ifdef 91, 232

if-else ambiguity 56, 223, 234 if-else

statement 19, 21, 55, 223 #ifndef 91,

232

illegal pointer arithmetic 102-103, 138, 205

implicit declaration of function 27, 72, 201

#include 33, 88, 152, 231

incomplete type 212 inconsistent type

declaration 72 increment operator, ++ 18,

46, 106, 203 indentation 10, 19, 23, 56

indirection operator, * 94, 203 inequality

operator, I= 16, 41, 207 infinite loop, for ( ; ; )

60, 89 information hiding 67-68, 75, 77

initialization 40, 85, 218 initialization, array

86, 113, 219 initialization by string constant

86, 219 initialization, default 86, 219

initialization in block 84, 223 initialization of

automatics 31, 40, 85, 219 initialization of

externals 40, 81, 85, 219 initialization of

statics 40, 85, 219 initialization of structure

arrays 133 initialization of two-dimensional

array 112,

220

initialization, pointer 102, 138

initialization, structure 128, 219

initialization, union 219 initializer

227

initializer, form of 85, 209

inode 179

input, buffered 170

input, formatted see scanf

input, keyboard 15, 151, 170

input pushback 78

input, unbuffered 170 input/output,

character 15, 151 input/output errors 164,

248 input/outputredirection 152,161,170

install function 145

int type 9, 36, 211

integer constant 12, 37, 193 integer-

character conversion 45 integer-

floating conversion 12, 197 integer-

pointer conversion 199, 205 integral

promotion 44, 197 integral types 196

INDEX

267

internal linkage 195, 228

internal names, length of 35, 192

internal static variables

IOFBF,

IOLBF,

IONBF 243

isalnumlibrary function

136, 249

isalpha library function

136, 166, 249

iscntrl library function

249

isdigit library function

166, 249

isgraph library function

249

islower library function

166, 249

ISO character set 229

isprint library function

249

ispunct library function

249

isspace library function

136, 166, 249

isupper library function

166, 249

i sxdigi t

library function

249

iteration statements

224

i toa function 64

jump statements 224

keyboard input 15, 151, 170

keyword count program 133

keywords, list of 192

label 65, 222

label, case 58, 222

label, default

58, 222

label, scope of 66, 222, 228

labeled statement

65, 222

labs library function

253

%ld conversion

ldexp library function 251

ldiv library function

253

leap year computation

41, Ill

left shift operator,

49, 206

length of names

35, 192

length of string

30, 38, I 04

length of variable names

192

less or equal operator, <=

41, 206

less than operator, < 41, 206

lexical conventions

191

lexical scope 227

lexicographic sorting

118

library function

7, 67, 80

<limits. h> header

36, 257

#line 233

line count program

__ LINE __ preprocessor name

254

line splicing 229

linkage 195, 227-228

linkage, external

73,

192, 195, 211, 228

linkage, internal

195, 228

list directory program

179

list of keywords

192

locale issues 241

<locale. h> header

241

local time library function

256

log, log 10 library functions

251

logical AND operator,&.&.

21, 41, 49, 207

logical expression, numeric value of

268 THE C PROGRAMMING LANGUAGE

logical negation operator, I 42, 203-204

logical OR operator, : J 21, 41, 49, 208 long

constant 37, 193

long double constant 37, 194 long

double type 36, 196 long type 10,

18, 36, 196, 211 longest-line

program 29, 32 longjmp library

function 254 LONG_~ LONG_MIN

252 lookup function 145 loop see

while, for, do

lower case conversion program 153

lower function 43

ls command 179

lseek system call 174

lvalue 197

macro preprocessor 88, 228-233

macros with arguments 89 magic

numbers 14 main function 6

main, return from 26, 164

makepoint function 130 malloc

function 187

malloc library function 143, 167, 252

manifest constant 230

<math.h> header 44, 250

member name, structure 128, 213

memchr library function 250

memcmp library function 250

memcpy library function 250

memmove library function 250

memset library function 250

missing storage class specifier 211

missing type specifier 211

mktime library function 256

modf library function 251

modularization 24, 28, 34, 67, 74-75, 108

modulus operator, " 41, 205

month_day function Ill month name

function 113 morecore function 188

multi-dimensional array 110, 217

multiple assignment 21 multiple

files, compiling 70 multiplication

operator, * 41, 205 multiplicative

operators 205 multi-way decision

23, 57

mutually recursive structures 140, 213

\n newline character 7, 15, 20, 37-38, 193, 241

name 192

name hiding 84

name space 227

names, length of 35, 192

negative subscripts 100

nested assignment statement 17, 21, 51

nested structure 129

newline 191, 229

newline character, \n 7, 15, 20, 37-38, 193, 241

INDEX

new-style function

202

NULL 102

null character, \0

30, 38, 193

null pointer l 02,

198

null statement 18, 222

null string 38

numbers, size of

9, 18, 36, 257

numcmp function

121

numeric sorting

118

numeric value of logical expression 44

numeric value of relational expression 42, 44

object 195, 197

octal character constant

octal constant, 0 ...

37, 193

old-style function

26, 33, 72, 202

one's complement operator, -

49, 203-204

open system call

172

opendir function

183

operations on unions

148

operations permitted on pointers

l 03

operators, additive

205

operators, arithmetic

operators, assignment

42, 50, 208

operators, associativity of

52, 200

operators, bitwise

48, 207

operators, equality

41, 207

operators, multiplicative

205

operators, precedence of

17, 52, 95, 131-132,

200

operators, relational

16, 41, 206

operators, shift

48, 206

operators, table of

order of evaluation

21, 49, 53, 63, 77, 90, 95,

200

order of translation

228

172

O_RDONLY, O_RDWR, O_WRONLY

output, formatted

see printf

output redirection

152

output, screen

15, 152, 163, 170

overflow 41, 200, 250, 255

parameter

84, 99, 202

parameter, definition of 25, 201

parenthesized expression

201

parse tree

123

parser, recursive-descent

123

pattern finding program

67, 69, 116-117

permissions, file 173

perror library function

248

phases, translation

191, 228

pipe 152, 170

pointer argument

100

pointer arithmetic

94, 98, 100-103, 117, 138,

205

pointer arithmetic, illegal

102-103, 138, 205

pointer arithmetic, scaling in

l 03, 198

pointer comparison

102, 138, 187,207

pointer conversion

142, 198, 205

pointer, declaration of

94, 100, 216

pointer, file

160, 175, 242

pointer generation

200

THE C PROGRAMMING LANGUAGE

pointer initialization 102, 138

pointer, null 102, 198

pointer subtraction 103, 138, 198

pointer to function 118, 147, 201

pointer to structure 136

pointer, void* 93, 103, 120, 199 pointer

vs. array 97, 99-100, 104, 113 pointer-

integer conversion 198-199, 205 pointers

and subscripts 97, 99, 217 pointers, array

of 107

pointers, operations permitted on 103

Polish notation 74

pop function 77

portability 3, 37, 43, 49, 147, 151, 153, 185

position of braces 10

postfix ++ and -- 46, 105 pow

library function 24, 251

power function 25, 27

lpragma 233

precedence of operators 17, 52, 95, 131-132,

200

prefix ++ and -- 46, 106

preprocessor, macro 88, 228-233

preprocessor name, __ FILE __

254

preprocessor name, __ LINE__

254

preprocessor names, predefined

233

preprocessor operator,# 90, 230

preprocessor operator,## 90, 230

preprocessor operator, defined 91, 232

primary expression 200

printd function 87

print£ conversions, table of 154, 244

print£ examples, table of 13, 154

print£ library function 7, 11, 18, 153, 244

printing character 249

program arguments see command-line

arguments

program, calculator 72, 74, 76, 158

program, cat 160, 162-163 program,

character count 18 program, del 125

program, echo 115-116 program, file

concatenation 160 program, file copy 16-

17, 171, 173 program format 10, 19, 23, 40,

138, 191 program, fsize 181

program, keyword count 133 program, line

count 19 program, list directory 179

program, longest-line 29, 32 program, lower

case conversion 153 program, pattern

finding 67, 69, 116-117 program readability

10, 51, 64, 86, 147 program, sorting 108,

119

program, table lookup 143

program, temperature conversion 8-9, 12-13, 15

program, undcl 126

program, white space count 22, 59

program, word count 20, 139

promotion, argument 45, 202

promotion, integral 44, 197

prototype, function 26, 30, 45, 72, 120, 202

ptinrect function 130

INDEX

ptrdiff_ t type name 103, 147, 206 push

function 77

pushback, input 78

putc library function 161, 247

putc macro 176

putchar library function 15, 152, 161, 247 puts

library function 164, 247

qsort function 87, 110, 120

qsort library function 253

qualifier, type 208, 211

quicksort 87, 110

quote character, ' 19, 37-38, 193

quote character. n 8, 20, 38, 194

\r carriage return character 38, 193

raise library function 255

rand function 46

rand library function 252

RAND MAX 252

read system call 170 readdir function 184

readlines function 109 realloc library

function 252 recursion 86, 139, 141, 182,

202, 269 recursive-descent parser 123

redirection see input/output redirection

expression, numeric value of 42, 44 relational

operators 16, 41, 206

removal of definition see #undef

remove library function 242

rename library function 242

reservation of storage 210

reserved words 36, 192

return from main 26, 164

return statement 25, 30, 70, 73, 225

return, type conversion by 73, 225

reverse function 62

reverse Polish notation

74 rewind library

function 248

Richards, M. 1 right

shift operator, »

Ritchie, D. M. xi

sbrk system call 187

scaling in pointer arithmetic 103, 198

scanf assignment suppression 157, 245

scanf conversions, table of 158, 246 scanf

library function 96, 157, 246 scientific

notation 37, 73

scope 195, 227-228

scope, lexical 227

scope of automatics 80, 228

scope of externals 80, 228

scope of label 66, 222, 228

scope rules 80, 227

screen output 15, 152, 163, 170

SEEK_C~ SEEK_END, SEEK_SET 248

selection statement 223

270THE C PROGRAMMING LANGUAGE

INDEX

self-referential structure

I40, 213

<stddef .h> header

103, 135, 241

semicolon I 0, I5, I8, 55, 57

stderr

16I, I63, 242

separate compilation

67, 80, 227

stdin

161, 242

sequencing of statements

222

<stdio. h> contents

176

setbuf library function

243

<stdio.h> header

6, 16, 89-90, 102,

setjmp library function

254

I51-152, 241

<setjmp.h> header

254

<stdlib.h> header

71, 142, 251

setvbuf library function

243

stdout

161, 242

Shell, D. L.

storage allocator 142, 185-189

shellsort function

storage class 19 5

shift operators 48, 206

storage class, automatic

31, 195

short type

10, 36, 196, 21I

storage class declaration

210

side effects

53, 90, 200, 202

storage class specifier

210

SIG DFL, SIG E~ SIG IGN 255

storage class specifier, auto

210

sign extension 44-45, 177.-193

storage class specifier, extern

31, 33, 80,

signal library function

255

210

<signal. h> header

255

storage class specifier, missing 211

signed character

44, I95

storage class specifier, register 83, 210

signed type 36, 211

storage class specifier, static

83, 210

sin library function

251

storage class, static

31, 83, 195

sinh library function

251

storage, definition of

210

size of numbers

9, I8, 36, 257

storage order of array

112, 217

size of structure

138, 204

storage, reservation of

210

sizeof operator 91, 103, 135, 203-204, 247

strcat function 48

size_t type name 103, 135, 147, 204, 242

strcat library function

249

sorting, lexicogr11phic

118

strchr library function

249

sorting, numeric

118

strcmp function 106

sorting program

I 08, 119

strcmp library function

249

sorting text lines

107, 119

strcpy function 105-106

specifier, auto storage class 210

strcpy library function

249

specifier, enum

39, 215

strcspn library function

250

specifier, extern storage class

31, 33, 80,

stream, binary I60, 241-242

210

stream, text 15, 151, 24I

specifier, missing storage class

211

strerror library function

250

specifier, register storage class 83, 210

strftime library function

256

specifier, static storage class

83, 210

strindex function

specifier, storage class

210

string concatenation

38, 90,

194

specifier, struct 212

string constant 7, 20, 30, 38, 99, I 04, 194

specifier, type 211

string constant, initialization by

86, 219

specifier, union

2I2

string constant, wide

194

splicing, line

229

string, length of 30, 38,

104

sprint£ library function

155, 245

string literal see string constant

sqrt library function

251

string, type of 200

squeeze function 47

<string.h> header

39, 106,249

srand function

strlen function 39, 99, 103

srand library function 252

strlen library function

250

sscanf library function

246

strncat library function

249

standarderror 161,170

strncmp library function

249

standard headers, table of

241

strncpy library function

249

standard input 151, 161, 170

strpbrk library function

250

standard output

I52, 161, 170

strrchr library function

249

stat structure

180

strspn library function

250

stat system call 180

strstr library function

250

statement terminator

10,

strtod library function

251

statements 222-225

strtok library function

250

statements, sequencing of

242

strtol, strtoul library functions 252

stat. h include file 180-181

struct specifier 212

133

static function declaration 83

structure arrays, initialization of

static storage class 31, 83, 19S

structure declaration

128, 212

static storage class specifier

83, 210

structure initialization

128, 219

static variables, external

structure member name

128, 213

static variables, internal

structure member operator,

128, 201

statics, initialization of 40, 85, 2I9

structure, nested 129

<stdarg. h> header

155, 17 4, 254

structure pointer operator, -> 131, 201

THE C PROGRAMMING LANGUAGE

INDEX

271

structure, pointer to 136

trigraph sequence 229

structure reference semantics 202

trim function 65

structure reference syntax

202

truncation by division

l 0, 41, 205

structure, self-referential

140, 213

truncation of floating point

45, 197

structure, size of 138, 204

two-dimensional array

ll 0,

112, 220

structure tag 128, 212

two-dimensional array, initialization of 112,

structures, arrays of

132

220

structures, mutually recursive 140, 213

type conversion by return

73, 225

subarray argument

l 00

type conversion operator

see cast

subscripting, array

22, 97, 201, 217

type conversion rules

42, 44,

198

subscripts and pointers

97, 99, 217

type declaration

216

subscripts, negative

100

type declaration, inconsistent

subtraction operator, -

41, 205

type equivalence

221

subtraction, pointer

103, 138, 198

type, incomplete

212

suffix, constant

193

type names 220

swap function

88, 96, 110, 121

type of constant

37, 193

switch statement

58, 75, 223

type of string

200

symbolic constants, length of 35

type qualifier

208, 211

syntax notation

194

type specifier

211

syntax of variable names

35, 192

type specifier, missing

211

syscalls. h include file

171

typedef declaration

146, 210, 221

system calls 169

types, arithmetic

196

system library function

167, 253

types, derived

l 0, 196

types, floating

196

ll, 38,

types, fundamental

9, 36, 195

'.t tab character 8,

193

types, integral

196

table lookup program

143

types.hincludefile

181,183

table of escape sequences

38, 193

table of operators

table of printf conversions 154, 244

ULONG_MAX

252

table of printf examples

13, 154

unary minus operator, -

203-204

table of scanf conversions

158, 246

unary plus operator, +

203-204

table of standard headers

241

unbuffered getchar

171

tag, enumeration 215

unbuffered input

170

tag, structure 128, 212

undcl program

126

tag, union

212

#undef

90, 172, 230

talloc function

142

underflow 41, 250, 255

tan library function

251

underscore character,

35, 192, 241

tanh library function

251

ungetc library function

166, 247

temperature conversion program 8-9, 12-13,

ungetch function

union, alignment by

186

tentative definition

227

union declaration

147, 212

terminal input and output

union initialization

219

termination, program

162, 164

union specifier

212

text lines, sorting

107, 119

union tag 212

text stream

15, 151, 241

unions, operations on

148

Thompson, K. L.

UNIX file system

169, 179

time library function

256

unlink system call

174

<time. h> header

255

unsigned char type

36, 171

time_t type name

255

unsigned character

44, 195

tmpfile library function

243

unsigned constant

37, 193

TMP MAX 243

unsigned long constant 37, 193

tmpnam library function

243

unsigned type

36, 50, 196, 211

token 191, 229

90, 230

usual arithmetic conversions

42, 198

token concatenation

token replacement

229

'.v vertical tab character

38, 193

tolower library function

153, 166, 249

toupper library function

166, 249

va_list,va_start,va_arg,va_end

translation, order of

228

155, 174, 245, 254

translation phases

191, 228

variable

195

translation unit

191, 225, 227

variable, address of

28, 94, 203

tree, binary

139

variable, automatic

31, 74, 195

tree, parse

123

variable, external

31, 73, 195

treeprint function

142

variable length argument list

155, 174, 202,

218,

225,

2~4

272 THE C PROGRAMMING LANGUAGE INDEX

variable names, length of 192

variable names, syntax of 35, 192

vertical tab character, \v 38, 193

void * pointer 93, 103, 120, 199

void argument list 33, 73, 218, 225

void type 30, 196, 199, 211

volatile qualifier 196, 211

vprintf, vfprintf, vsprintf library

functions 174, 245

wchar _ t type name 193

while statement 10, 60, 224

while vs. for 14, 60

white space 191

white space characters 157, 166, 245, 249

white space count program 22, 59

wide character constant 193

wide string constant 194

word count program 20, 139

write system call 170

writelines function 109

\x hexadecimal escape sequence 37, 193

zero, omitted test against 56, 105

The C Programming Language

Second Edition

Brian W. Kernighan/Dennis M. Ritchie

From the Preface

We have tried to retain the brevity of the first edition. C is not a big language, and it

is not well served by a big book. We have improved the exposition of critical

features, such as pointers, that are central to C programming . We have refined the

original examples, and have added new examples in several chapters. For instance,

the treatment of complicated declarations is augmented by programs that convert

declarations into words and vice versa . As before, all examples have been tested

directly from the text, which is in machine-readable form.

As we said in the preface to the first edition, C "wears well as one' s experience with

it grows . " With a decade more experience , we still feel that way . We hope that this

book will help you to learn C and use it well.

PRENTICE HALL, Englewood Cliffs, N.J. 07632

ISBN 0-13-110362-8